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Chapter 1

Introduction to Hyperbranched Polymers

1.1 INTRODUCTION

According to IUPAC Compendium, polymers are defined as large molecules

composed of basic building blocks known as monomers which repeat throughout the

structure. In 1833, Jöns Jacob Berzelius coined the term polymer from the Greek word

poly, meaning “many” and mer, meaning “parts” (McNaught and Wilkinson 1997).

Branching, which is more useful in transferring, dissipating and distributing

energy and /or matter occurs anywhere and anytime such as trees, veins, nerves, crab, etc

from light years to kilometers and micrometers to nanometers in nature and universe

(Yan et al 2010).

1.2 POLYMER ARCHITECTURE

Since the past century, scientists like Staudinger, Flory, Ziegler, Natta, de Gennes,

Shirakawa, Heeger, MacDiamid, Noyori, Sharpless, Grubbs and many others have made

remarkable contributions towards the development of Polymer Science and Technology.

Notably, their focus was mainly concentrated on linear polymer chains. In 1920,

Staudinger presented several reactions that form high molecular weight molecules by

linking together a large number of small molecules in his paper entitled “Uber

Polymerisation” (Yan et al 2010). He called this process as “polymerization” in which

individual repeating units are joined together by covalent bonds which lead to the concept

of macromolecules. Since then, numerous types of macromolecules with various

architectures have been synthesized successfully (Fig. 1.1). Except linear and interlocked

polymers, all other architectures possess branched structures which indicate the

significance of branching in the construction of molecules (Yan et al 2010).

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Fig. 1.1: Architectures of synthesized macromolecules: (a) linear, (b) chain-branched, (c)

cross-linked, (d) cyclic, (e) star-like, (f) ladder-like, (g) dendritic, (h) linear brush-like, (i)

cyclic brush-like, (j) sheet-like, (k) tube-like, and (l) interlocked (Yan et al 2010).

1.3 DENDRITIC POLYMERS

Dendritic polymers which came into being in the 1980s are classified as

macromolecules with tree-like 3-D structures with a large number of branching units and

became one of the most interesting topics of research in the area of Polymer Science and

Engineering. These macromolecules are being considered as the fourth major class of

polymers in addition to linear, cross-linked and chain-branched polymers. Dendritic

architecture which has been widely studied and industrially used, consists of six

subclasses (Fig. 1.2): (a) dendrons and dendrimers; (b) linear-dendritic hybrids; (c)

dendrigrafts or dendronized polymers; (d) hyperbranched polymers; (e) multi-arm star

polymers and (f) hypergrafts or hypergrafted polymers. The first three subclasses exhibit

perfect and ideally branched structures with a degree of branching (DB) of 1.0, while the

latter three exhibit a random and irregular branched structure with lesser DB (~0.4 to 0.6)

(Gao and Yan 2004; Yan et al 2010). Since the present work is concerned with

hyperbranched polyesters, only the synthesis, physical properties and applications of

hyperbranched polymers are reviewed in the Introduction section.

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Fig. 1.2: Classification of dendritic polymers.

1.4 HYPERBRANCHED POLYMERS

1.4.1 INTRODUCTION

Kim and Webster (1988 & 1990) coined the term “Hyperbranched Polymers” as

polymer systems containing a large number of branchings connected by a relatively short

chains resulting in a highly branched tree-like structures with 3-D dendritic architecture.

Although in the beginning, hyperbranched polymers were not considered to be promising

materials due to their broad molecular weight, non-entangle/non-crystalline behavior and

poor mechanical properties, they gained due importance after the discovery of dendrimers

with unique properties in the mid 1980s (Flory 1952; Hult et al 1998; Jikei et al 1999).

Despite the fact that dendrimers have a well-defined structure, they require step-wise

synthesis, whereas hyperbranched molecules can be synthesized by one-step method.

Hence, hyperbranched polymers started to become an alternative to dendrimers because

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of their relative ease of preparation and lower cost. Hyperbranched molecules are

believed to have properties similar to dendrimers eventhough they possess some defects

on the polymer backbone (Maelstrom and Hult 1996).

The unique nonlinear structure and more number of end functional groups which

could be tailor-made into different hyperbranched polymers with specific physical and

chemical properties have attracted increasing attention over the last two decades. As a

result, there is an increase in the number of publications every year regarding synthesis,

properties and applications of hyperbranched polymers. Because of its nonlinear

structure, the individual molecules can have different molecular weight and degrees of

branching. Their compact globular 3-D structure and the presence of large number of

functional groups at the periphery result in different properties when compared to that of

linear analogues and consequently leading to different applications.

Several excellent reviews have been written on the history and current trends in

hyperbranched polymers (Voit 2005; Yates and Hayes 2004; Gao and Yan 2004; Ishizu

et al 2002; Seiler 2002; Jikei and Kakimoto 2001; Inoue 2000; Voit 2000; Kim 1998;

Seiler 2006). The forthcoming discussions provide a brief insight into the history,

synthesis, properties and applications of hyperbranched polymers.

1.4.2 HISTORY OF HYPERBRANCHED POLYMERS

For the first time in the history, in 1909, Bakelite Company (U.S.A) introduced

cross-linked phenolic resins, obtained from formaldehyde (latent A2 monomer) and

phenol (latent B3 monomer). These polymers have a so-called random hyperbranched

structure before gel formation (Gao and Yan 2004). Berzelius reported the formation of

resin by reacting tartaric acid as A2B2 monomer and glycerol as B3 monomer (Kienle and

Hovey 1929). Kinele et al (1939) also reported the condensation reaction between

glycerol and phthalic anhydride, a basic synthetic methodology which is followed even

nowadays for the synthesis of hyperbranched polymers. This reaction when continued

beyond the stipulated time resulted in gel formation due to crosslinking. Kinele further

studied this reaction and other similar systems and observed that the specific viscosity of

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the obtained polymer from the reaction between phthalic anhydride and glycerol is low

when compared to that of linear polymers such as polystyrene.

The concept of 'highly branched species' was introduced by Flory in the 1940s

(Flory 1941). He studied the molecular weight distribution in 3-D polymers with

trifunctional and tetrafunctional branching units and formulated the critical conditions for

gelation and also applied statistical treatment to deduce the molecular composition. In

another study, Flory (1952) synthesized highly branched polymers without gelation by

polycondensation of ABn monomers (n ≥ 2) in which A and B functional groups can react

with each other.

Kim and Webster (1988) coined the term “Hyperbranched Polymer” when they

intentionally prepared the soluble hyperbranched polyphenylenes while working in

DuPont. Since then, hyperbranched polymers have acquired significant attention from

industry and academia owing to their unique properties and ease of preparation in large

quantity and greater availability while compared to that of dendrimers.

1.4.3 SYNTHESIS METHODOLOGY

The synthesis of hyperbranched polymer could be divided into two major

categories namely (a) single-monomer methodology (SMM) in which hyperbranched

polymers are synthesized by polymerization of an ABn or a latent ABn monomer and (b)

double-monomer methodology (DMM) in which direct polymerization of two types of

monomers or a monomer pair generates hyperbranched polymers (Gao and Yan 2004).

SMM follows four different approaches (Yates and Hayes 2004; Gao and Yan

2004; Jikei and Kakimoto 2001):

(1) Polycondensation of ABn monomers resulting in a wide range of

hyperbranched polymers such as polyphenylenes, polyesters, polyamides and

polycarbonates.

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(2) Self-condensation of AB* type vinyl monomers to synthesize polystyrenes,

poly(methyacrylate) or poly(acrylates)s. This method is also classified as Self

Condensing Vinyl Polymerization (SCVP).

(3) Self-Condensing Ring Opening Polymerization (SCROP) of latent ABn

monomers type to obtain polyamines, polyethers and polyesters and

(4) Proton-Transfer Polymerization (PTP) of hyperbranched polysiloxanes or

polyesters with epoxy or hydroxyl end groups.

DDM can be classified into two main subclasses depending on the selection of

monomer pairs and pathways of the reaction:

(1) ‘A2 + B3’ methodology to synthesize polycarbonates, polyamides and

polyureas.

(2) Couple-monomer methodology (CMM), a combination of the basic SMM and

‘A2 + B3’ methodology, is employed to synthesize hyperbranched polymers such as

poly(urea urethane)s, poly(sulfone amine)s and poly(ester amine)s.

1.4.4 PROPERTIES OF HYPERBRANCHED POLYMERS

One of the many intriguing physical properties of hyperbranched polymers is their

considerable lower viscosity than that of linear analogues of the same molar mass. The

viscosity of hyperbranched polymers in solution as well as in molten state is lower than

that of linear analogues (Yates and Hayes 2004). This is due to less entanglement because

of their globular shapes. In addition to having globular conformations, it also possesses

branching structure. Higher the degree of branching, lower the viscosity of the polymer.

The degree of branching architecture in hyperbranched polymer indicates the flexibility

of the branching components (Yates and Hayes 2004).

The nature and number of end groups of hyperbranched polymers significantly

affect the final properties of the polymers such as glass transition temperature and

solubility in various solvents. In general, hyperbranched polymers exhibit excellent

solubility in most of the organic solvents. Due to weaker chain entanglements,

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dendrimers and hyperbranched polymers possess inferior mechanical characteristics. As a

result, dendritic polymers exhibit brittle nature which limits its use as thermoplastics

(Kim and Webster 1988), whereas their stress-strain behavior is similar to that of ductile

metals (Rogunova et al 2000). Hyperbranched polymers have a controllable size of 2-15

nm (Yan et al 2010).

Based on the property of the solvents, polymers exhibit variable hydrodynamic

radii. The molecular weight of hyperbranched polymers as well as dendrimers determined

by SEC using linear polymers such as polystyrene, polyethyleneoxide and

polyvinylpyridine standards must be regarded with some skepticism (Appelhans et al

2000; Lederer et al 2002). This is due to smaller hydrodynamic radii of hyperbranched

polymers than those of their linear analogs with the same molar mass. The hydrodynamic

radii were also influenced based on the polarity of functional groups on the end groups.

Among the thermal properties, glass transition temperature (Tg) is the most

important property to be studied. Hyperbranched polymers are generally amorphous in

nature owing to their highly branched structure. Amorphous polymers change from

glassy state to liquid state for low molar mass substances or from glassy to rubbery state

for high molar mass compounds when the polymer is heated up. In the melt state, the

each polymer chain of longer segments undergo Brownian motion whereas, this motion is

ceased below Tg. The segmental motions in hyperbranched polymers are restricted

because of branching in it. Unlike linear analogues, hyperbranched polymers exhibit

significantly different thermal properties as discussed in many articles (Hult et al 1999;

Voit 2000; Voit 2005; Benthem 2000; Behera 2005; Hawker and Chu 1996). Tg of

hyperbranched polymers depends on the backbone of the structure and end groups

(Wooley et al 1993). The second order transition increases with increasing generation

numbers to a limit, above which it becomes plateau (Wooley et al 1993). The increase in

Tg with increasing generation number is owing to decrease in the chain mobility due to

highly branched structure.

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1.4.5 APPLICATIONS OF HYPERBRANCHED POLYMERS

Hyperbranched polymers and its modified polymers have varying solubility,

compatibility, thermal stability, reactivity, electrochemical, luminescence properties, etc.

Thus, hyperbranched polymers and its modified polymers have a wide range of

applications in many fields such as ranging from engineering to nano and medical fields

due to their unique properties and easy method of synthesis.

Because of low viscosity characteristics, high solubility and more number of end

functional groups, it could be used as blend components or rheology modifiers (Kim and

Webster 1992; Hong et al 2000; Jang et al 2000; Mulkern and Tan 2000), excellent

toughener for epoxy matrix composites (Mezzenga et al 2001; Xu et al 1999; Wu et al

1999; Boogh et al 1999) and as a base material in coating industry such as high solid

coatings (Mancyck and Szewczyk 2002), powder coatings (Johansoon et al 2000;

Benthem 2000) and flame retardant coatings (Zhu and Shi 2002).

In addition to that, it is used in nanocomposite preparation (Wang et al 2006;

Zhao et al 2007; Plummer et al 2005; Rodlert et al 2004) with carbon nanotubes and is

utilized as an electrode material for lithium batteries (Wang et al 2006). In the field of

Chemical Engineering, it is employed in solvent/metal-ion extraction process and as

membranes (Seiler et al 2004; Zou et al 2004; Sterescu et al 2008; Goswami and Singh

2004).

Due to their 3-D structure, outstanding processibility and better solubility, the

conjugated hyperbranched polymers such as poly(phenylenevinylene) and

polythiophenes were used as novel optical (Lin et al 2000; He et al 2001; Zhang et al

1996), magnetic (Spetseris et al 1998) and electronic (Duan et al 2001; He et al 2002)

materials.

Being amorphous in nature, it could be used as polymer electrolytes by using

ethylene glycol chains and they possess good solvating power for appropriate ions, better

ion transport and electrochemical stability (Hawker et al 1996; Itoh et al 1999; Itoh et al

2002; Wen et al 2000; Itoh et al 2003; Wen et al 2000; Hong et al 2002; Wang et al 2001;

Nishimoto et al 1999).

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Hyperbranched polymers and their modified polymers can be utilized as

nanomaterials for host-guest encapsulation (Sunder et al 1999; Stiriba et al 2002) for the

production of organic-inorganic hybrids (Plummer et al 2002; Sun et al 2000) and as

nanoreactors (Gao and Yan 2004). In addition, it can be used as sensors (Gao and Yan

2004) and multifunctional crosslinkers (Maji and Bhowmick 2009).

Since hyperbranched polymers having multifunctional terminal groups and

narrow polydispersity, they could be synthesized by one-shot method with ease in large

quantity at low-cost, and have received a lot of attention in biomaterials such as

biodegradable materials as well as biocarriers (Frey and Haag 2002; Lim et al 2001). The

guest molecules can be sequestered in the presence of functional groups on end groups as

well as on the inner shells of the hyperbranched polymers which fix bioobjects

covalently.

1.5 THESIS OUTLINE

In the present thesis, the research work is mainly divided into four chapters

dealing with the synthesis of hyperbranched polyesters (HBPs) by CMM (Xn + AB2) with

varying core moieties but with the same monomer (DMPA), their characterization and

their applications. The main objectives of each chapter are described briefly as follows:

The present chapter is dealt with the introduction, classification of polymer

architectures as well as dendritic polymers. It also dwells into the general introduction,

history, synthesis methodology, properties and applications of hyperbranched polymers.

Finally, it concludes with the objective of the thesis.

Chapter 2 involves the synthesis and characterization of first to third generation of

HBPs (HBP-G1 to HBP-G3) using phenyl dichlorophosphate, PDCP as a core and

DMPA as a monomer. It also examines the suitability of HBP-G3 as a crosslinker in

polyurethane curing kinetics and determines the order of reaction as well as deriving the

thermodynamic parameters.

Chapter 3 investigates the effect of varying generations of HBPs (HBP-G1 to

HBP-G4) on toughening the epoxy resin using HBP as a modifier. Chemical blending of

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HBP with epoxy resin is carried out through the formation of graft interpenetrating

polymer networks (g-IPNs) by the reaction between hexamethylene diisocyanate, HBP

and the epoxy resin. Prior to the toughening of epoxy resin by HBPs, their synthesis

using dipentaerythritol as a core and DMPA as a monomer by pseudo-one-step melt

polycondensation method and their characterizations are carried out.

Chapter 4 describes the synthesis of third generation HBP using pentaerythritol as

a core and DMPA as a monomer by pseudo-one-step melt polycondensation method and

its characterization. The effect of HBP content on the toughening of epoxy resin will be

studied by reacting toluene diisocyanate, HBP with epoxy resin through the formation of

graft interpenetrating polymer networks (g-IPNs).

Chapter 5 investigates the miscibility of pentaerythritol based HBP -

polyvinylidene fluoride (HBP/PVDF) blends as a function of varying weight ratios.

Changes in the PVDF crystalline modification as a function of different spin coating

temperatures are studied using heat-controlled spin-coating setup upon varying substrates

such as KBr, ITO and Gold-coated glass slides. Based on the above results, further

studies are carried out to compare the effect of thermal annealing (30 oC to 200

oC) upon

PVDF and PVDF/HBP blend (90/10 w/w) using FTIR-GIRAS and the effect of heating-

cooling cycle using FTIR-TS and GIRAS.

Chapter 6 provides a conclusive discussion on the research work carried out.