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Polymer 55 (2014) 6132e6139

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Polymer

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Tailoring the rigid amorphous fraction of isotactic polybutene-1 byethylene chain defects

Maria Laura Di Lorenzo a, *, Ren�e Androsch b, *, Isabell Stolte b

a Consiglio Nazionale delle Ricerche, Istituto per i Polimeri, Compositi e Biomateriali, c/o Comprensorio Olivetti, Via Campi Flegrei, 34, 80078 Pozzuoli, NA,Italyb Martin-Luther-University Halle-Wittenberg, Center of Engineering Sciences, D-06099 Halle/Saale, Germany

a r t i c l e i n f o

Article history:Received 22 July 2014Received in revised form11 September 2014Accepted 17 September 2014Available online 28 September 2014

Keywords:Isotactic polybutene-1Rigid amorphous fractionRandom copolymers

* Corresponding authors.E-mail addresses: dilorenzo@ictp.cnr.it (M.L. Di Lor

halle.de (R. Androsch).

http://dx.doi.org/10.1016/j.polymer.2014.09.0400032-3861/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

The effect of different amounts of ethylene co-units in the butene-1 chain, on the fold-surface structureof crystals of isotactic polybutene-1, has been probed by analysis of the rigid amorphous fraction (RAF).The exclusion of ethylene co-units from crystallization in random butene-1/ethylene copolymers andtheir accumulation at the crystal basal planes leads to a distinct increase of the RAF with increasingconcentration of co-units. A specific RAF was determined by normalization of the RAF to the crystalfraction. While in the butene-1 homopolymer a specific RAF of 20e30% is detected, it increases to morethan 100% in copolymers with 5e10 mol% of ethylene co-units, being in accordance with the previouslyobserved increase of the free energy of the crystal fold-surface due to copolymerization. It has also beenshown that the specific RAF increases with decreasing temperature of crystallization, due to formation ofa fold-surface of lower perfection, containing an increased number of chain segments traversing thecrystalline-amorphous interface.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Semicrystalline polymers consist of crystalline and amorphousphases. The latter can be subdivided into a rigid amorphous fraction(RAF) and a mobile amorphous fraction (MAF), based on the degreeof coupling with the crystal phase, affecting the chain mobility [1].The MAF is made of chain segments which are decoupled from thecrystals and mobilize at their glass transition temperature (Tg,MAF).The RAF has a glass transition temperature (Tg,RAF) higher thanTg,MAF, and is covalently coupled to the crystals, as it arises from thecontinuation of the partially crystallized macromolecules acrossthe phase boundaries. For several polymers, including poly (L-lacticacid) (PLLA), poly [(R)-3-hydroxybutyrate] (PHB), and poly(ethylene terephthalate) (PET) it has been proven that vitrificationof the RAF on cooling, and devitrification on heating, is indis-pensably connected with the formation and melting of crystals,respectively [2e4].

The importance to quantify the RAF in semicrystalline polymersderives from the fact that the RAF is glassy at temperatures higherthan Tg,MAF, which influences thermo-mechanical properties

enzo), rene.androsch@iw.uni-

[5e10], and that it is of lower density than the MAF, which affectsthe barrier properties [11e13]. Recent analyses even suggested thatvitrification of the RAF may be the cause for early termination ofcrystallization [14]. Ordering of macromolecules drastically slowsdown as soon as the vitrified RAF hinders the diffusion of chainsegments towards the growing crystals. The hindering effect of RAFvitrification on crystallization was proven for poly (ε-caprolactone)(PCL), isotactic polypropylene (iPP) and PHB [15e17].

Analysis of the effect of the crystal habit on the RAF of iPP, whichcan be crystallized to contain either nodular or lamellar crystals oflargely different ratio of the areas of lateral and fold surfaces,allowed identification of the structure at the basal planes of crystalsas a major source of the immobilization of the amorphous phase[9]. Furthermore, specific annealing experiments performed on PETrevealed that the local RAF at the crystal basal planes is reducedwith increasing perfection/regularity of the fold-surface [18]. Be-sides the crystallization history, the structure of the crystal basalplanes, and with that of the RAF, may be controlled by the chemicalcomposition of macromolecules. With the present study it isattempted to demonstrate that the presence of a small amount offoreign co-units in a crystallizable macromolecule, which disruptthe lateral growth of crystals due to their exclusion from the crys-tallization process and accumulate then at the crystal basal planes,is a tool to tailor the RAF and, with that, the ultimate properties.

M.L. Di Lorenzo et al. / Polymer 55 (2014) 6132e6139 6133

We selected a series of random copolymers of butene-1 withlow amount of ethylene co-units (up to 10 mol%) in order to test thehypothesis of tailoring the RAF by modification of the structure ofcrystal basal planes. It has been shown that ethylene co-units inrandom butene-1/ethylene copolymers are excluded from crystal-lization, as was proven by analysis of the composition-dependenceof the lattice parameter a0, measured by wide-angle X-ray diffrac-tion [19e22]. In the case of butene-1/propylene random co-polymers, the presence of propylene co-units in the butene-1chains causes a significant reduction of the unit cell dimension incross-chain direction, whereas in the case of ethylene co-units anegligible effect is observed. It is assumed that the propylene chaindefects are incorporated into the crystals, with the decrease of theaverage distance between neighbored chain segments related tothe smaller size of the eCH3 group of the propylene co-unit, incomparison to that of the eCH2eCH3 side group of butene-1 co-units. Conversely, ethylene co-units are excluded frompoly(butene-1) crystals due to their small size [23]. As a conse-quence, in random butene-1/ethylene copolymers the maximumcrystallinity decreases and the kinetics of the transformation of thesupercooled melt into tetragonal Form II crystals decreases withincreasing molar percentage of ethylene co-units [21,22]. Such adecrease is not detected in butene-1/propylene copolymers, wherethe kinetics of melteForm II transition is independent of compo-sition, at least for a content of propylene co-units up to 11 mol %[24]. The exclusion of ethylene co-units from crystallization ofbutene-1 chain segments, and their enrichment at the crystal fold-surface has been confirmed by quantitative analysis of fold-surfacefree energies which, in copolymers containing up to 5 mol%ethylene is 50e100% higher than in the butene-1 homopolymer[23]. In contrast, incorporation of either ethylene or propylene co-units into the butene-1 chains leads to an acceleration of the rate ofthe polymorphic transformation of the unstable Form II phase intostable trigonal Form I crystals [19,24,25].

The formation of an RAF was proven for isotactic polybutene-1(iPB-1) [7,26]. It amounts to 15e25%, depending on crystallizationtemperature (Tc), and on the Form II/Form I crystal polymorphism[27e30]. It has been shown that the RAF in semicrystalline iPB-1decreases with Tc, and increases after Form II e Form I trans-formation. Note that the density of Form I crystals is about 4%higher than that of Form II crystals, which may cause an increase ofthe local stress at the crystal surfaces due to the phase transition,and an increase of the RAF.

In summary, the goal of this contribution is the establishment ofa link between the chemical structure of macromolecules, themorphology/fold-surface structure of crystals, and the RAF. Anadditional aim is to complete prior work on the crystallization ki-netics and the semicrystalline morphology of random butene-1/ethylene copolymers, and to explore in particular the influence ofethylene content not only on the crystal fraction, but on the overallthree-phase structure as function of copolymer composition andcrystallization conditions. This study we consider being of

Table 1List of isotactic random butene-1/ethylene copolymers used in this work, includinginformation about the content of ethylene co-units and the mass-average molarmass.

Trade name [33] Sample code Ethylene contentmol% m% [25]

Molar mass kg mol�1

[25,34]

PB 0300M iPB-Eth 0e347 0 0 347PB 0110M iPB-Eth 0e711 0 0 711PB 8340M iPB-Eth 1.5e293 1.5 0.75 293PB 8640M iPB-Eth 1.5e470 1.5 0.75 470PB 8220M iPB-Eth 4.3e400 4.3 2.2 400DP 8310M iPB-Eth 10.5e305 10.5 5.5 305

importance not only to advance the current knowledge on thethree-phase structure of semicrystalline polymers, but also as aneffort to permit further tailoring of structure-property relations ofrandom butene-1/ethylene copolymers, which have a large eco-nomic importance, being used e.g. as component in the seal layer ofeasy-opening packaging films [31e33].

2. Experimental

Random isotactic butene-1/ethylene copolymers were obtainedfrom Lyondell Basell (Germany). Table 1 is a list of the homo- andcopolymers used in this work, including information about theconcentration of ethylene co-units and the mass-average molarmass [25,34]. The as-received sample chips were processed to filmsof 500 mm thickness by compression-molding using a Per-kineElmer FTIR press in combination with a Lot-Oriel/Specac filmmaker die and heating accessory.

Thermal analysis was conducted with a PerkineElmer PyrisDiamond DSC, equipped with an Intracooler II as cooling system.The instrument was calibrated regarding temperature with highpurity standards (indium and cyclohexane) and regarding energyby the heat of fusion of indium. Dry nitrogenwas used as purge gasat a rate of 48 ml min�1.

To analyze the effect of crystallization conditions on RAF for-mation, isothermal crystallization experiments were performed.The compression-molded samples were heated to 180 �C at a rate of20 K min�1, maintained at this temperature for a period of 2 min inorder to destroy all traces of previous crystalline order, and thenrapidly cooled to the desired crystallization temperature at a rate of100 K min�1, to allow crystallization. The isothermally crystallizedsamples were then rapidly cooled to�68 �C, and heated to 180 �C ata rate of 20 K min�1.

In order to obtain precise heat-capacity values, the experimen-tally measured heat-flow-rate raw data were corrected for instru-mental asymmetry by subtraction of a baseline, measured underidentical conditions as the samples, including close match of themasses of the aluminum pans. The heat-flow rate data were thenconverted to specific apparent heat capacities by point-by-pointcalibration with sapphire [35]. All measurements were repeatedthree times to improve accuracy.

The RAF of the polymers listed in Table 1 was determined as afunction of the conditions of melt-crystallization by analysis ofcrystalline fraction Xcry and of the MAF, according to Equation (1):

RAF ¼ 1�MAF� Xcry (1)

The MAF was obtained by the heat-capacity increment Dcp,measat Tg,MAF on heating of the semicrystalline sample, and itsnormalization to the expected heat-capacity step Dcp,100 for a fullyamorphous sample, according to Equation (2):

MAF ¼ Dcp;meas�Dcp;100 (2)

The heat-capacity step Dcp,100 of a fully amorphous sample is thedifference between the heat capacities of the glassy and liquidphases of the sample under consideration, cp,solid and cp,liquid,respectively. Both, cp,solid and cp,liquid depend on the copolymercomposition and were calculated for the iPBeEth copolymers byaveraging of the heat capacities of polybutene-1 (iPB-1) and poly-ethylene (PE), taking into account their molar fraction n in thecopolymers, as shown by Equation (3a) and (3b) [36e39]:

cp; solid; iPBeEth ¼ cp; solid; iPB�1 � nbutene þ cp; solid; PE � nethylene(3a)

M.L. Di Lorenzo et al. / Polymer 55 (2014) 6132e61396134

cp; liquid; iPBeEth ¼ cp; liquid; iPB�1 � nbutene þ cp; liquid; PE

� nethylene (3b)

The heat capacities of the iPB-1 and PE (in J mol�1 K�1) weretaken from the literature [40], then the heat capacities of the co-polymers, determined with Equation (3a) and (3b), were convertedto J g�1 K�1 to simplify presentation of the results. The crystallinefraction in Equation (1) was obtained by the measured enthalpy ofisothermal crystallization, Dhcry, normalized with the enthalpy ofcrystallization Dhcry,100, as would be obtained for a fully crystal-lizing sample:

Xcry ¼ Dhcry�Dhcry;100 (4)

We employed values of 62 and 141 J g�1 for normalization ofmeasured enthalpies of crystallization, in case of formation of FormII and Form I crystals, respectively [41]. Further details of analysis ofthe RAF by calorimetry were published elsewhere [19,42].

3. Results and discussion

Fig. 1 shows apparent specific heat capacities of the iPB-1 ho-mopolymer with a molar mass of 347 kg mol�1 as function oftemperature, measured on heating at a rate of 20 K min�1. Prior theheating experiment, samples were isothermally crystallized atdifferent temperatures between 82 and 100 �C and then rapidlycooled to �68 �C. The dotted and dashed lines are the specific heatcapacities of solid and liquid iPB-1, respectively, as taken from theATHAS Data Bank [40]. The inset is an enlargement of the curves inthe temperature range of Tg,MAF, showing the heat-capacity stepdue to devitrification of the MAF, required for calculation of thethree-phase composition.

The DSC curves of Fig. 1 reveal with the heat-capacity step ofclose to 0.1 J g�1 K�1, centered at �25 �C, the glass transition of theMAF. For fully amorphous iPB-1 the heat-capacity change is

Fig. 1. Apparent specific heat capacity of the iPB-1 homopolymer with a molar mass of347 kg mol�1 as function of temperature, obtained on heating at 20 K min�1. Beforeheating, the samples were isothermally crystallized at different temperatures between82 �C (blue curve) and 100 �C (red curve) and cooled to �68 �C. The inset is anenlargement in the temperature range of Tg,MAF, for illustration of the heat-capacitystep on devitrification of the MAF. The dotted and dashed lines are the specific heatcapacities of solid and liquid iPB-1, as taken from the ATHAS Data Bank [40]. (Forinterpretation of the references to colour in this figure legend, the reader is referred tothe web version of this article.)

expected to be 0.5 J g�1 K�1. As such the MAF is less than 0.2, ac-cording to Equation (2), and the remaining part of the sample re-mains solid on continuation of heating. The MAF glass transition isthen followed by a broad andweak exothermic peak between 0 and5 �C, which could be due to cold-crystallization or transition ofForm II into Form I crystals, presently under investigation. Onfurther heating, the experimental heat-capacity data remain belowthat of liquid iPB-1 until the onset of fusion. Two endothermicmelting peaks are detected. The main melting peak at about 115 �Cis due to fusion of iPB-1 crystals of Form II which were grown uponcrystallization from the melt. The smaller peak at about 135 �C isrelated to the melting of a small portion of Form I crystals whichconverted from Form II crystals mainly during heating. The obser-vation of a small amount of Form I crystals in iPB-1, not subjected toaging, is often observed in commercial iPB-1 grades due to thepresence of nucleating agents that have been added to theformulation in order to accelerate the Form II e Form I phasetransition [7]. Both melting peaks move to higher temperatureswith increasing Tc, which reveals not only the expected increasedthermodynamic stability of the tetragonal Form II crystals grown athigher temperatures, but also that the Form I crystals transformedfrom Form II modification exhibit higher thermodynamic stabilitywhen the initial step of the crystallization is conducted at highertemperatures. Thermal analysis of the iPB-1 grade with a molarmass of 711 kg mol�1 discloses similar results as presented in Fig. 1.

Incorporation of ethylene co-units into the butene-1 chain leadsto a depression of both the equilibrium melting temperature ofcrystals [43e45], and of the glass transition temperature of the bulkamorphous phase, ultimately leading to a decrease of the temper-ature range of crystallization [21e23]. The thermodynamicallycontrolled exclusion of ethylene chain defects from the crystalli-zation process leads furthermore to a reduction of the maximumrate of formation of Form II crystals from the melt [21,22], and,presumably, to a change of the morphology of crystals. It isassumed that ethylene chain defects enrich at the basal plane ofcrystals according the Flory model of copolymer crystallization[43,44], as it is in accordwith the experimentally proven increase ofthe surface free energy of crystals due to the presence of ethyleneco-units in the butene-1 chains [22]. The change of both the crys-tallization kinetics and the crystal morphology leads to character-istic changes of the DSC heating scans, recorded after isothermalcrystallization, shown in Fig. 2. Specifically, Fig. 2 shows apparentspecific heat capacity of random copolymers of butene-1 with (a)1.5, (b) 4.3, and (c) 10.5 mol% ethylene as function of temperature,obtained on heating at 20 K min�1. Before heating, samples wereisothermally crystallized at different temperatures in the rangedefined by the blue and red curves in each set of curves, and cooledto �68 �C. As in Fig. 1, the dotted and dashed lines are the specificheat capacities of solid and liquid random butene-1/ethylene co-polymers, calculated with Equation (3).

Visual inspection of the three sets of curves reveals a minordecrease of Tg, MAF and a distinct increase of Dcp,meas with theconcentration of ethylene co-units. Furthermore, there is observeda remarkable decrease of the crystal fraction Xcry, as is concludedfrom the lowered total area of the endothermic peaks, recentlyquantified in a parallel study [22]. Note again that quantitativeanalysis of Dcp,meas and Xcry is required for estimation of the RAF,discussed below with respect to variation of the crystallizationtemperature and ethylene co-unit concentration.

The data of Fig. 2 additionally reveal, with the observed decreaseof the melting temperatures of Form II and Form I crystals, indi-cated with the black solid and dashed lines, respectively, the ex-pected decrease of the temperature range of crystallization withincreasing co-unit concentration. It is important to note that thepresence of ethylene co-units in the butene-1 chain leads to an

Fig. 3. Crystalline fraction of the iPB-1 homopolymer (blue symbols) and randomcopolymers of butene-1 with (a) 1.5 (red symbols), (b) 4.3 (green symbols), and (c)10.5 mol% ethylene (dark gray symbols) as function of the isothermal crystallizationtemperature. The different sets of data points for the homopolymer and for thecopolymer with 1.5 mol% ethylene were obtained on samples of different molar mass.(For interpretation of the references to colour in this figure legend, the reader isreferred to the web version of this article.)

Fig. 2. Apparent specific heat capacity of random copolymers of butene-1 with (a) 1.5,(b) 4.3, and (c) 10.5 mol% ethylene as function of temperature, obtained on heating at20 K min�1. Before heating, the samples were isothermally crystallized in a tempera-ture range which is indicated with the blue and red curves in each set of curves, andcooled to �68 �C. The dotted and dashed lines are the specific heat capacities of solidand liquid random butene-1/ethylene copolymers, calculated according to Equation(3). (For interpretation of the references to colour in this figure legend, the reader isreferred to the web version of this article.)

M.L. Di Lorenzo et al. / Polymer 55 (2014) 6132e6139 6135

acceleration of the Form II to Form I phase transition [19,25]. Whilecompletion of the Form II to Form I phase transition in the iPB-1homopolymer requires aging for several days, in copolymers withsufficiently high co-unit concentration, the transition may becompleted after few hours. In other words, the transition occurs atthe time scale of the DSC experiment, that is, during the coolingthat follows the isothermal growth of Form II crystals, and duringthe subsequent heating. In the DSC scans of the copolymers, thepresence of Form I crystals is then detected with the high-temperature melting peak. Regarding the effect of variation of thecrystallization temperature on the rate of transformation of Form IIinto Form I crystals, as is judged by the ratio of the peak areas of thelow- and high-temperature melting peaks, the various samplesbehave different. While for the homopolymer and the copolymerswith 1.5, and 4.3 mol% ethylene the crystallization temperature hasonly a negligible effect on the ratio of the peak areas of the low- andhigh-temperature melting peaks, that is, on the rate of trans-formation of Form II into Form I crystals, in case of the copolymerwith 10.5 mol% ethylene it is detected that the transformation isfaster if the crystallization temperature is lowered.

For all investigated samples, including the iPB-1 homopolymer,exothermic heat flow is detected on heating at temperatures

around 0 �C. The peak area increases with the concentration ofethylene co-units; however, only in case of the copolymer with10.5 mol% ethylene a distinct effect of the crystallization temper-ature is observed. With decreasing crystallization temperature, theexothermic peak around 0 �C increases in area, and simultaneouslythe peak area of the high-temperature Form I melting peak in-creases on expense of the low-temperature Form II melting peak.This observation suggests that the exothermic peak around 0 �C isdue to the transition of Form II into Form I crystals, though directformation of Form I' crystals from the melt cannot be excluded.Note that direct formation of Form I crystals from the melt, termedForm I' crystals [46,47], bypassing the intermediate formation ofForm II crystals, has been suggested a possible route of crystalli-zation of random butene-1 copolymers containing large amount ofco-units [19,48].

The observation of multiple melting events on heating thecopolymer with 10.5 mol% ethylene has been explained in priorwork to be due to the presence of crystals of different thermody-namic stability, forming according to the distribution of the lengthof crystallizable butene-1 chain segments at various temperatures,and due to crystal reorganization on slow heating at 20 K min�1

[25] which, however, is not object of the present work.Fig. 3 shows the crystalline fraction Xcry of the homo- and co-

polymers listed in Table 1 as a function of the isothermal crystal-lization temperature. Due to the complexity of the DSC heatingscans recorded after prior isothermal crystallization, in particular incase of the copolymers containing 4.3 and 10.5 mol% ethylene, aprecise determination of crystallinity is not straightforward. Diffi-culties arise from the presence of both Form II and Form I crystalpolymorphs of largely different specific heat of crystallization/melting [41] and by definition of the heat-capacity baseline forintegration of the excess heat capacities. As such, the crystal frac-tion Xcry was calculated from the exothermic heat flow recordedduring isothermal crystallization, assuming that only Form IIcrystals developed. This assumption is justified since for the iPB-1homopolymer and the copolymer containing 1.5 mol% ofethylene, the DSC analysis conducted immediately after isothermalcrystallization and quenching revealed only a minor amount ofForm I crystals. In contrast, the DSC heating scans obtained on thecopolymers containing 4.3 and 10.5 mol% of ethylene units showedpresence of a considerable fraction of Form I crystals, which,

M.L. Di Lorenzo et al. / Polymer 55 (2014) 6132e61396136

however, formed mainly during heating, as confirmed by analysisof the melting behavior of samples crystallized for different times,as well as by thermal analysis of the same copolymer cooled andheated at different rates [49]. Only a negligible endothermic peakdue to melting of Form I crystals appears after 30 min of isothermalcrystallization at 20 �C, that is, at the temperature of maximumForm II to Form I transformation rate [25], in the copolymer con-taining 10.5 mol% of ethylene. Variation of the crystallization time,within the ranges investigated in the present study, does not resultin a sizeable change of ratio of the areas of the melting peaksassociated to presence of Form II to Form I crystals. In other words,if a very small fraction of Form II crystals transforms into Form Imodification during isothermal crystallization, which may occuronly for the grades containing high ethylene content, the errorappears minor compared to that arising from integration of thecomplex DSC plots shown in Fig. 2. The cooling rate employed totransfer the sample from the crystallization temperature to belowTg,MAF is sufficiently high to prevent additional crystallization aswell as distinct Form II to Form I crystal transformation duringcooling. The latter mainly occurs during the subsequent heating at20 K min�1 [49].

Integration of the exothermic crystallization peak to determinethe crystalline fraction was performed following the general rec-ommendations described in textbooks [50], however, with a specialprocedure applied to expand the temperature range for isothermalcrystallization analysis [51e53]. Depending on the crystallizationtemperature, the crystallization-induced exothermic sample-heatflow may overlap with an instrumental drift related to the switchfrom the cooling segment to the isothermal segment of thetemperature-time program. In this case, the initial stage of thephase transition proceeds with the instrument in a non-steadystate, requiring a correction of the recorded total heat-flow ratesignal for precise determination of the enthalpy of crystallization.For estimation of the instrumental drift, the crystallized samplewas subjected to an identical rate of temperature change as on theapproach of the isothermal crystallization temperature, by heatingto a temperature above the melting point, and then isothermallyheld at this temperature. Due to absence of change of latent heat,the recorded signal provides information about the approach ofinstrumental steady state in presence of the specific sample, but notoverlapped by non-steady sample-heat flow. An example is illus-trated in Fig. 4, for isothermal crystallization of iPB-1 homopolymerat 82 �C.

Fig. 4. Heat-flow rate plots of iPB-1: experimental raw data during isothermal crys-tallization at 82 �C (black line), experimental raw data recorded above the meltingtemperature in absence of phase transition (red line), corrected heat-flow rate data(blue line). (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

The data of Fig. 3 reveal that the crystal fraction in case of theiPB-1 homopolymer crystallized at the highest crystallizationtemperature of 100 �C is close to 0.65, being similar as wasobserved after cooling at 1 K min�1 in an independent study [21].Decreasing the crystallization temperature to 82 �C results in thecrystallinity to be slightly reduced by about 5%. Presence ofethylene co-units in the butene-1 chain leads to a distinct reductionof the maximum crystallinity which ultimately is caused by thedisruption of the chain regularity and the shortening of butene-1chain segments. Note that in case of incorporation of chain de-fects into the crystalline phase the reduction of the crystallinity isconsiderably less [22]. The remaining fraction [1�Xcry] is amor-phous phase consisting of MAF and RAF. The MAF is calculated fromthe heat-capacity step Dcp,meas at Tg,MAF and comparison with thevalue Dcp,100 expected for a fully amorphous material (see Equation(2)). Visual inspection of the data of Fig. 2 reveals that an increase ofthe ethylene content in the butene-1 chain is connected with anincrease of Dcp,meas, indicating mobilization of a larger fraction ofamorphous material at Tg,MAF.

Calculation of the RAF according to Equation (1) yields infor-mation about its absolute amount in the various samples. For theiPB-1 homopolymers, the RAF is about 20% of the total samplemass, while it slightly increases to 25 and 30% in case of the co-polymers containing 1.5 and 4.3 mol% ethylene. In case of thecopolymer with 10.5 mol% ethylene, the RAF is between 25 and30%. It is worthwhile noting that an increase of the RAF withincreasing amount of ethylene co-units in the random copolymersis not straightforward expected, since establishment of the RAF isindispensably coupled to the formation and presence of crystals. Ithas been shownwith the data of Fig. 3 that insertion of ethylene co-units into the butene-1 chain leads to a distinct reduction of thecrystallinity, which may have led to the conclusion that the RAFdecreases correspondingly. The latter is not observed since the totalRAF, calculated by Equation (1), even slightly increases despite thelargely reduced crystallinity.

Fig. 5 is a plot of the specific RAF of iPB-1 homopolymers (bluesymbols) and random copolymers of butene-1 with (a) 1.5 (redsymbols), (b) 4.3 (green symbols), and (c) 10.5 mol% ethylene (darkgray symbols) as function of the isothermal crystallization tem-perature. The specific RAF is obtained by normalization of the totalRAF with the crystallinity [ ¼ RAF/Xcry], and provides informationabout the average RAF per unit crystal [18]. As in Fig. 3, the differentsets of data points for the homopolymer and for the copolymer

Fig. 5. Specific RAF of iPB-1 homopolymers (blue symbols) and random copolymers ofbutene-1 with (a) 1.5 (red symbols), (b) 4.3 (green symbols), and (c) 10.5 mol%ethylene (dark gray symbols) as function of the isothermal crystallization temperature.The different sets of data points for the homopolymer and for the copolymer with1.5 mol% ethylene were obtained on samples of different molar mass. (For interpre-tation of the references to colour in this figure legend, the reader is referred to the webversion of this article.)

M.L. Di Lorenzo et al. / Polymer 55 (2014) 6132e6139 6137

with 1.5 mol% ethylene were obtained on samples of differentmolar mass. The data of Fig. 5 suggest for the iPB-1 homopolymersa specific RAF of about 0.3 [ ¼ RAF/Xcry ¼ 0.2/0.6]. In other words, ifwe assume that crystal and rigid amorphous fractions have thesame density, a lamella with a thickness of 10 nmwould be coveredat its top and bottom fold-surfaces by an RAF layer with a thicknessof 1.5 nm each. It must be underlined that such assumption is notstrictly correct, since the RAF has a somewhat lower density thanthe crystal fraction, but the exact value is at moment unknown.Based on the data available for other semicrystalline polymers [1],we can roughly hypothesize that the density of the RAF is about10e15% lower than that of the crystals, which provides an esti-mation of the error associated to the thickness ratio of the twolayers. The specific RAF increases distinctly with increasing contentof ethylene co-units in the copolymers, to reach values suggestinglayer thicknesses similar as the thickness of crystals. For example,in case of the copolymer with 4.3 mol% ethylene, the specific RAF isaround 1.3 [ ¼ RAF/Xcry ¼ 0.325/0.25], that is, for a crystal with athickness of 10 nm the thickness of the immobilized amorphouslayers at the top and bottom surfaces is around 6.5 nm each.

The observation of an increase of the specific RAF withdecreasing temperature of crystallization for a given sample (seeFig. 5) is explained by the increase of the crystal growth rate withdecreasing temperature, which increasingly hinders the formationof regular fold-surfaces. In other words, crystallization at hightemperatures improves crystal perfection and reduces the couplingbetween crystalline and amorphous structures [54]. This result is inaccord with former fundamental studies of the RAF in PET, iPP, PHB,and PA 6, which revealed that the specific RAF in semicrystallinepolymers decreases in samples of high crystallinity, typically ach-ieved by crystallization at high temperature [9,18,54,55].

Fig. 6 is an illustration of the relative thickness of the immobi-lized rigid-amorphous layer (gray) at the fold-surfaces of crystals ofiPB-1 (left) and a random copolymer of butene-1 with 4.3 mol%ethylene (right). In case of the iPB-1 homopolymer, the thickness ofthe rigid-amorphous layer is only about 25% of the crystal thick-ness, suggesting largely decoupled crystalline and amorphousphases, achieved by formation of a rather regular fold-surface withonly few chain segments traversing the crystalline-amorphousphase boundary. The formation of perfect fold-surfaces is hin-dered in random butene-1/ethylene copolymers, due to the

Fig. 6. Illustration of the relative thickness of the immobilized rigid-amorphous layer(gray) at the fold-surfaces of crystals of iPB-1 (left) and a random copolymer of butene-1 with 4.3 mol% ethylene (right). The sketch at the right-hand side shows that ethyleneco-units (red circles) are excluded from crystallization and enrich at the crystalline-amorphous interface. The fold-surface free energies se have been determined in anindependent study by HoffmaneLauritzen analysis of crystal growth rates [23]. (Forinterpretation of the references to colour in this figure legend, the reader is referred tothe web version of this article.)

thermodynamically enforced exclusion of ethylene chain defectsfrom the crystallization process [19e26]. Growth of Form II crystalsproceeds via attachment of defect-free molecular stems at the{100} faces [56]. The ethylene co-units will therefore accumulate atthe fold surface, but not at the lateral faces of the crystals. As aconsequence, compared to the iPB-1 homopolymer, the fold-surface free energy increases by 50e100%, due to generation oflocal dilatometric stress caused by the distinctly lower size of theethylene co-units compared to butene-1 co-units. This interpreta-tion of the increase of the fold-surface free energy is in agreementwith former studies of the crystallization behavior of random co-polymers [57e61]. It has been suggested that surface energiesgenerally increase with co-unit content, since a larger number ofchain segments must participate on the growth of a single crystal ofa given size, as compared with the corresponding homopolymer[60], ultimately leading to enhanced covalent coupling of theamorphous phase to the crystals by the increased number of chainsegments traversing the phase boundary.

Furthermore, regarding the sketches of the local structure at thefold-surface of crystals in the iPB-1 homopolymer and randombutene-1/ethylene copolymer in Fig. 6, it is worthwhile noting thatthe RAF layer has been drawn with a rather sharp boundary to theMAF, as it is emphasized with the dashed line in the sketch at theright-hand side. The DSC heating scans of Figs. 1 and 2 showed thatthe glass transition of the MAF is not significantly broadened to-wards higher temperature, suggesting that theMAF and RAF shouldbe considered as separate nanophases of different properties, andnot parts of a single phase in which the restriction of the chainmobility is exponentially decreasing with the distance from thecrystal surface [42,62].

Fig. 7 shows the specific RAF of iPB-1 homopolymers andrandom butene-1/ethylene copolymers as a function of theethylene co-unit concentration. The color-coding of the symbols isin analogy to Fig. 5, with the different symbols plotted at a given co-unit concentration referring to the variation of the crystallizationtemperature and to samples of different molar mass. The data ofFig. 7 show that the RAF increases, as explained above, with thecontent of ethylene co-units in the random butene-1/ethylene co-polymers, to asymptotically approach a maximum value at a co-unit concentration of 5e10 mol%. It may be speculated that theleveling of the specific RAF at high co-unit concentration is relatedto a maximum number of ethylene chain defects which can beaccommodated at the fold surface. In such case, the requiredsegregation of ethylene co-units at the crystal growth front could

Fig. 7. Specific RAF of iPB-1 homopolymers and random butene-1/ethylene co-polymers as a function of the ethylene co-unit concentration. The color-coding of thesymbols is in analogy to Fig. 5, with the different symbols plotted at a given co-unitconcentration referring to the variation of the crystallization temperature and tosamples of different molar mass. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

M.L. Di Lorenzo et al. / Polymer 55 (2014) 6132e61396138

be achieved by a drastic shortening of the lateral crystal size, whichhas been proven valid for different random copolymers requiringexclusion of co-units from crystallization, studied by microscopy[63e67].

4. Conclusions

In the present study, it has been shown on example of thecopolymer system butene-1/ethylene that the rigid amorphousfraction, located at the fold-surface of crystals in semicrystallinepolymers, can be tailored by incorporation of non-crystallizable co-units in random crystallizable copolymers. It is known that the RAFis a measure of the number of molecule segments traversing thecrystalline-amorphous interface, that is, of the degree of covalentcoupling of crystals and amorphous phase. The formation of regularfold-surfaces with low number of traversing chain segments ishindered in random copolymers with non-crystallizable chain de-fects, since a larger number of chain segments must participate inthe growth of crystals of a given size, as compared with the cor-responding homopolymer. This leads to enhanced covalentcoupling of the amorphous phase to the crystals and, as a conse-quence, to a distinct increase of the RAF per crystal unit. In thespecific case of the butene-1 homopolymer, the rigid-amorphouslayer thickness is only 20e30% of the crystal thickness. In the co-polymers, the rigid-amorphous layer thickness tremendously in-creases due to the presence of 5e10 mol% ethylene co-units in thebutene-1 chain, to reach a thickness of 50e75% of the thickness ofthe crystals. The experimental observation of an increase of theRAF, as was probed by analysis of the relaxation strength of theMAFand precise determination of the crystalline fraction, and itsinterpretation in terms of an increased immobilized rigid-amorphous layer at the fold-surface of crystals, is confirmed in anindependent study of the fold-surface free energy. It has beenobserved that the fold-surface free energy increases by 50e100%due to presence of rather low amount of ethylene-co-units, point-ing to their enrichment at the fold-surface and generation of localdilatometric stress due to the different size of ethylene and butene-1 co-units.

The RAF remains glassy at temperatures higher than the glasstransition temperature of the MAF and significantly affects thethermo-mechanical behavior and barrier properties of semi-crystalline polymers. With the present work, we provided a routeto tailor the RAF and with that ultimate properties of semi-crystalline polymers, by random copolymerization with non-crystallizable co-units.

Acknowledgments

Financial support by the Deutsche Forschungsgemeinschaft(DFG) (Grant AN 212/12) and the Italian National Research Council(CNR) Short-Term Mobility Program is greatly acknowledged.

References

[1] Wunderlich B. Reversible crystallization and the rigid amorphous phase insemicrystalline macromolecules. Prog Polym Sci 2003;28:383e450.

[2] Righetti MC, Tombari E. Crystalline, mobile amorphous and rigid amorphousfractions in poly(L-lactic acid) by TMDSC. Thermochim Acta 2011;522:118e27.

[3] Righetti MC, Tombari E, Di Lorenzo ML. The role of the crystallization tem-perature on the nanophase structure evolution of poly[(R)-3-hydroxybutyrate]. J Phys Chem B 2013;117:12303e11.

[4] Righetti MC, Laus M, Di Lorenzo ML. Temperature dependence of the rigidamorphous fraction in poly(ethylene terephthalate). Eur Polym J 2014;58:60e8.

[5] El Shafee E. Effect of aging on the mechanical properties of cold-crystallizedpoly(trimethylene terephthalate). Polymer 2003;44:3727e32.

[6] Rastogi R, Wellinga WP, Rastogi S, Schick C, Meijer HEH. The three-phasestructure and mechanical properties of poly(ethylene terephthalate).J Polym Sci Part B Polym Phys 2004;42:2092e106.

[7] Di Lorenzo ML, Righetti MC. The three-phase structure of isotactic poly(1-butene). Polymer 2008;49:1323e31.

[8] Bai H, Luo F, Zhou T, Deng H, Wang K, Fu Q. New insight on the annealinginduced microstructural changes and their roles in the toughening of b-formpolypropylene. Polymer 2011;52:2351e60.

[9] Zia Q, Mileva D, Androsch R. Rigid amorphous fraction in isotactic poly-propylene. Macromolecules 2008;41:8095e102.

[10] Kolesov I, Mileva D, Androsch R, Schick C. Structure formation of polyamide 6from the glassy state by fast scanning chip calorimetry. Polymer 2011;52:5156e65.

[11] Lin J, Shenogin S, Nazarenko S. Oxygen solubility and specific volume of rigidamorphous fraction in semicrystalline poly(ethylene terephthalate). Polymer2002;43:4733e43.

[12] Guinault A, Sollogoub C, Ducruet V, Domenek S. Impact of crystallinity ofpoly(lactide) on helium and oxygen barrier properties. Eur Polym J 2012;48:779e88.

[13] Delpouve N, Stoclet G, Saiter A, Dargent E, Marais S. Water barrier propertiesin biaxially drawn poly(lactic acid) films. J Phys Chem B 2012;116:4615e25.

[14] Wunderlich B. Termination of crystallization or ordering of flexible, linearmacromolecules. J Therm Anal Calorim 2012;109:1117e32.

[15] Zhuravlev E, Schmelzer JWP, Wunderlich B, Schick C. Kinetics of nucleationand crystallization in poly( 3-caprolactone) (PCL). Polymer 2011;52:1983e97.

[16] Mileva D, Androsch R, Zhuravlev E, Schick C, Wunderlich B. Homogeneousnucleation and mesophase formation in glassy isotactic polypropylene.Polymer 2012;53:277e82.

[17] Di Lorenzo ML, Gazzano M, Righetti MC. The role of the rigid amorphousfraction on cold crystallization of poly(3-hydroxybutyrate). Macromolecules2012;45:5684e91.

[18] Androsch R, Wunderlich B. The link between rigid amorphous fraction andcrystal perfection in cold-crystallized poly(ethylene terephthalate). Polymer2005;46:12556e66.

[19] Turner Jones A. Cocrystallization in copolymers of a-olefins IIdbutene-1 co-polymers and polybutene type II/I crystal phase transition. Polymer 1966;7:23e59.

[20] Stolte I, Androsch R. Kinetics of the melt e form II phase transition in isotacticrandom butene-1/ethylene copolymers. Polymer 2013;54:7033e40.

[21] Stolte I, Androsch R. Comparative study of the kinetics of non-isothermal meltsolidification of random copolymers of butene-1 with either ethylene orpropylene. Coll Polym Sci 2014;292:1639e47.

[22] Stolte I, Di Lorenzo ML, Androsch R. Spherulite growth rate and fold surfacefree energy of the form II mesophase in isotactic polybutene-1 and randombutene-1/ethylene copolymers. Coll Polym Sci 2014;292:1479e85.

[23] De Rosa C, Auriemma F, Ruiz de Ballesteros O, Resconi L, Camurati I. Crys-tallization behavior of isotactic propylene-ethylene and propylene-butenecopolymers: effect of comonomers versus stereodefects on crystallizationproperties of isotactic polypropylene. Macromolecules 2007;40:6600e16.

[24] Stolte I, Cavallo D, Alfonso GC, Portale G, van Drongelen M, Androsch R. FormI' crystal formation in random butene-1/propylene copolymers as revealed byreal-time X-ray scattering using synchrotron radiation and fast scanning chipcalorimetry. Eur Polym J 2014;60:22e32.

[25] Azzurri F, Alfonso GC, G�omez MA, Marti MC, Ellis G, Marco C. Polymorphictransformation in isotactic 1-butene/ethylene copolymers. Macromolecules2004;37:3755e62.

[26] Di Lorenzo ML, Righetti MC, Wunderlich B. Influence of crystal polymorphismon the three-phase structure and on the thermal properties of isotacticpoly(1-butene). Macromolecules 2009;42:9312e20.

[27] Danusso F, Gianotti G. The three polymorphs of isotactic polybutene-1 :dilatometric and thermodynamic fusion properties. Makromol Chem1963;61:139e56.

[28] Danusso F, Gianotti G. Isotactic polybutene-1: formation and transformationof modification 2. Makromol Chem 1965;88:149e58.

[29] Boor Jr J, Mitchell JC. Kinetics of crystallization and a crystal-crystal transitionin poly-1-butene. J Polym Sci Part A 1963;1:59e84.

[30] Androsch R, Di Lorenzo ML, Schick C, Wunderlich B. Mesophases in poly-ethylene, polypropylene, and poly(1-butene). Polymer 2010;51:4639e62.

[31] Nase M, Androsch R, Langer B, Baumann HJ, Grellmann W. Effect of poly-morphism of isotactic polybutene-1 on peel behavior of polyethylene/polybutene-1 peel systems. J Appl Polym Sci 2008;107:3111e8.

[32] Nase M, Funari SS, Michler GH, Langer B, Grellmann W, Androsch R. Structureof blown films of polyethylene/polybutene-1 blend. Polym Eng Sci 2010;50:249e56.

[33] Product information, lyondell basell. 2013. Available from: https://polymers.lyondellbasell.com/portal/site/basell/polybutene-1.

[34] Hadinata C. Flow-induced crystallization of polybutene-1 and effect of mo-lecular parameters. PhD Thesis. Melbourne. 2007.

[35] Archer DG. J Phys Chem Ref Data 1993;22:1441e53.[36] Di Lorenzo ML, Zhang G, Pyda M, Lebedev BV, Wunderlich B. Heat capacity of

solid-state biopolymers by thermal analysis. J Polym Sci Polym Phys 1999;37:2093e102.

[37] Androsch R, Wunderlich B. Thermochim Acta 1999;333:27e32.[38] Di Lorenzo ML, Pyda M, Wunderlich B. Reversible melting in nanophase-

separated poly(oligoamide-alt-oligoether)s and its dependence on sequence

M.L. Di Lorenzo et al. / Polymer 55 (2014) 6132e6139 6139

length, crystal perfection, and molecular mobility. J Polym Sci Polym Phys2001;39:2969e81.

[39] Pyda M, Di Lorenzo ML, Pak J, Kamasa P, Buzin A, Grebowicz J, et al. Reversibleand irreversible heat capacity of poly[carbonyl(ethylene-co-propylene)] bytemperature-modulated calorimetry. J Polym Sci Polym Phys 2001;39:1565e77.

[40] In: Pyda M (editor). ATHAS data bank. Web address: http://www.prz.rzeszow.pl/athas/databank/intro.html.

[41] Alfonso GC, Azzurri F, Castellano M. Analysis of Calorimetric curves detectedduring the polymorphic transformation of isotactic Polybutene-1. J ThermAnal Calor 2001;66:197e207.

[42] Androsch R, Wunderlich B. Scanning calorimetry. In: Matyjaszewski K,Gnanou Y, Leibler L, editors. Macromolecular Engineering: precise synthesis,materials properties, applications. Structure-property correlation and char-acterization techniques, Vol. 3. Weinheim, Germany: Wiley-VCH; 2007.

[43] Flory PJ. Thermodynamics of crystallization in high polymers. IV. A theory ofcrystalline states and fusion in polymers, copolymers, and their mixtures withdiluents. J Chem Phys 1949;17:223e51.

[44] Flory PJ. Theory of crystallization in copolymers. Trans Faraday Soc 1955;51:848e57.

[45] Sanchez IC, Eby RK. Thermodynamics and crystallization of random co-polymers. Macromolecules 1975;8:638e41.

[46] Boor Jr J, Youngman EA. Polymorphism in poly-1-butene: apparent directformation of modification I. J Polym Sci Polym Lett 1964;2:903e7.

[47] Rakus JP, Mason CD. The direct formation of modification I'polybutene-1.J Polym Sci Polym Lett 1966;4:467e8.

[48] Androsch R, Hohlfeld R, Frank W, Nase M, Cavallo D. Transition from two-stage to direct melt-crystallization in isotactic random butene-1/propenecopolymers. Polymer 2013;54:2528e34.

[49] Di Lorenzo ML, Androsch R, Righetti MC. On the irreversible tetragonal totrigonal transformation in random butene-1/ethylene copolymers. 2014 [tobe submitted].

[50] Wunderlich B. Thermal analysis. San Diego: Academic Press; 1990.[51] Righetti MC, Munari A. Influence of branching on melting behavior and

isothermal crystallization of poly(butylene terephthalate). Macromol ChemPhys 1997;198:363e78.

[52] Righetti MC, Di Lorenzo ML, Angiuli M, Tombari E, La Pietra P. Poly(butyleneterephthalate)/poly(ε-caprolactone) blends: Influence of PCL molecular masson PBT melting and crystallization behavior. Eur Polym J 2007;43:4726e38.

[53] Vyazovkin S, Chrissafis K, Di Lorenzo ML, Koga N, Pijolat M, Roduit B, et al.ICTAC Kinetics Committee recommendations for collecting experimentalthermal analysis data for kinetic computations. Thermochim Acta 2014;590:1e23.

[54] Di Lorenzo ML, Righetti MC. Effect of thermal history on the evolution ofcrystal and amorphous fractions of poly[(R)-3-hydroxybutyrate] upon storageat ambient temperature. Eur Polym J 2013;49:510e7.

[55] Kolesov IS, Androsch R. The rigid amorphous fraction of cold-crystallizedpolyamide 6. Polymer 2012;53:4770e7.

[56] Holland VF, Miller RL. Isotactic polybutene-1 single crystals: Morphology.J Appl Phys 1964;35:3241e8.

[57] Wunderlich B. Macromolecular physics. Crystal melting, vol. 3. New York:Academic Press; 1980.

[58] Howard PR, Crist B. Unit cell dimensions in model ethylene-butene-1 co-polymers. J Polym Sci Polym Phys 1989;27:2269e82.

[59] Mandelkern L. The relation between structure and properties of crystallinepolymers. Polym J 1985;17:337e50.

[60] Domszy RC, Alamo R, Mathieu PJM, Mandelkern L. The structure of copolymercrystals formed from dilute solution and in bulk. J Polym Sci Polym Phys1984;22:1727e44.

[61] Darras O, S�egu�ela R. Surface free energy of the chain-folding crystal faces ofethylene-butene random copolymers. Polymer 1993;34:2946e50.

[62] Ma Q, Georgiev G, Cebe P. Constraints in semicrystalline polymers: usingquasi-isothermal analysis to investigate the mechanisms of formation andloss of the rigid amorphous fraction. Polymer 2011;52:4562e70.

[63] Bensason S, Minick J, Moet A, Chum S, Hiltner A, Baer E. Classification ofhomogeneous ethylene-octene copolymers based on comonomer content.J Polym Sci Polym Phys 1996;34:1301e15.

[64] Mathot VBF, Scherrenberg RL, Pijpers TFJ. Metastability and order in linear,branched and copolymerized polyethylenes. Polymer 1998;39:4541e59.

[65] Kolesov IS, Androsch R, Radusch H-J. Effect of crystal morphology and crys-tallinity on the mechanical a- and b-relaxation processes of short-chainbranched polyethylene. Macromolecules 2005;38:445e53.

[66] Mileva D, Androsch R, Cavallo D, Alfonso GC. Structure formation of randomisotactic copolymers of propylene and 1-hexene or 1-octene at rapid cooling.Eur Polym J 2012;48:1082e92.

[67] Jeon K, Palza H, Quijada R, Alamo RG. Effect of comonomer type on thecrystallization kinetics and crystalline structure of random isotactic propylene1-alkene copolymers. Polymer 2009;50:832e44.