A Study on the Glass Transition Behavior and Morphology of Semi-Interpenetrating Polymer Networks

YlNC LI* and SUFEN M A 0

Department of Polymer Science and Engineering, School of Chemical Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi, 710049, China

SYNOPSIS

The epoxy resin/polyurethane semi-IPN was prepared and the glass transition behavior of the semi-IPN was discussed with DSC and DMA methods. The results show that the two glass transition temperatures (T,) referring to epoxy resin and polyurethane respectively get closer. Between the two T, there exists another Tg related to the interface between the two polymers. SEM indicates that this semi-IPN has a two-phase continuous structure which changes with different weight compositions. This is also proved by testing the Young's modulus. It is found that the IPN system has a cellular structure. Comparatively, the compatibility between the two polymers is the best when the weight ratio of EP/PU is 70/30. 0 1996 John Wiley & Sons, Inc. Keywords: epoxy resin polyether polyurethane interpenetrating polymer networks glass transition morphology

INTRODUCTION EXPERIMENTAL

Interpenetrating polymer networks (IPNs) are one type of polymer blend which combines the properties of the components forming the net- work. Since 1960 when Millar first put forward the term IPN, applications of I P N technology have reached indu~trial-scale. ' -~ One of the types of IPNs in which polyurethane (PU) is intro- duced is especially important because it has the advantage of a soft linear structure of polyure- thane.5

The semi-IPN of epoxy resin/polyurethane (EP/PU) was prepared and characterized. The semi-IPN is composed of a crosslinked polymer network and a linear polymer network. The pur- pose of this article is to discuss the glass transi- tion behavior and morphology of the EP /PU semi-IPN.

Materials

EP-014 epoxy resin and polyether polyurethane were chosen as two components. They can be described by the following:

W CH3

EP-014: molecular weight 2000-2400

0 0 0 II 0

Polyether polyurethane

Diaminodiphenylmethane (DDM) was used as cur- ing agent and dimethyl formamide (DMF) was used

* To whom all correspondence should be addressed. .Journal of Polymer Science: Part A Polymer Chemistry, Vol. 34,2371-2375 (1996) 0 1996 John Wiley L Sons, Inc. ccc 0887-624X/96/122371-05 as solvent.

2371

2372 LI AND M A 0

..'.. . ._

..,.., . . ....... ............. 1

. . . . . ...... ." 1 . . . . .

.._ . . . .

................ 3 . . . . ..._. . .

.. , . . . .(. '. * 4 ..'. . . .

. . . . . . ........ 5 _.

., .- ....

__..

.. ,

t l l l l l l l l l l t l l l l l l 1 1 1 t l l 1 1 L -

-120 -70 -20 30 s o 1

Temperature ( "(: ) Figure 1. DMA traces of EP/PU semi-IPNs with dif- ferent weight compositions: (1) 0/100, (2) 30/70, (3) 50/ 50, (4) 70/30, (5) 100/0.

Preparation

The IPNs were prepared by a two-step process. After dissolving the EP-014 and PU in DMF a t some ratio, we added DDM to the mixture while stirring. Then the mixture was poured into a polytetrafluoroeth- ylene mold, volatilized, and heated to 150-160°C. The blend was cured a t this temperature for 1 h. Then we obtained the EP/PU semi-IPN.

Methods

The glass transition behaviors of the semi-IPN were detected by TA2000 differential scanning calorim- eter (DSC) a t 10"C/min and MFIFA-1 multifunc- tion internal friction apparatus of dynamic me- chanical analysis (DMA) at 1 Hz, 3"C/min. The morphology of the free surface and the fracture sur- face of the semi-IPN was observed by S-2700 scan- ning electronic microscope (SEM).

RESULTS AND DISCUSSION

Glass Transition Behavior

The glass transition of the semi-IPN was detected at low temperature and a t high temperature by DMA

L, L 3

1 1 I 1

70 120 170

Temperature ( )

Figure 2. DSC traces of EP/PU semi-IPNs with dif- ferent weight compositions: (2) 30/70, (3) 50/50, (4) 70/ 30, ( 5 ) 100/0.

and DSC methods, respectively (Figs. 1 and 2). The glass transition can respond to the compatibility be- tween two polymers.

From curve 1 in Figure 1, one can see tha t P U has two glass transition temperatures (Tg), as Table 1 presents. The one a t -88°C corresponds to the flexible segments in PU, the other a t -5°C corresponds to the rigid segments in PU. The microphase separation between the flexible and rigid segments leads to a concentration gra- dient of rigid segments which serve as crosslinking points for soft segments. Such a type of structure is helpful to improve the mechanical properties of PU.

Table I. T, Data of Semi-IPNs

0/100 -88 -5 30/70 -80 20 44 120

70/30 -72 24 54 112 50/50 -82 19 46 125

100/0 -48 138

SEMI-INTERPENETRATING POLYMER NETWORKS 2373

(a) 70/30 (b) 50/50

(c)3 0/70 Figure 3. 70/30, (b) 50/50, (c) 30/70.

SEM micrographs of the EP/PU semi-IPNs with different weight ratios: (a)

In Figure 2, the change a t 138°C corresponds to the motion of long segments of E P chains which responds to the glass transition of EP. Another change a t -18°C in Figure 1 corresponds to the sec- ond-order transition of amorphous parts of EP, such as the motions of short sections of the main chain or of side chains.

Comparing several DSC and DMA curves, we notice that the glass transition of PU in the semi- IPN takes place a t higher temperature than that of pure PU. This is because the interactions be- tween PU molecular chains and E P crosslinked network increase the steric hindrance of the main chain’s motion. In contrast to PU the Tg of E P moves to lower temperature. This also asso-

ciates with the interpenetrating between PU soft linear chains and E P crosslinked network. Due to the interpenetration, the distance between E P molecular chains increases and thus the steric hindrance of motion lowers. Moreover, the in- terpenetration can make PU segments fill the de- fects in E P network and improve the properties of polymers.

One can see that when the weight composition of EP/PU is 70/30, Tg peaks of two polymers are the closest. This concludes that E P and PU have the best compatibilities a t this ratio.

In addition, between the peaks of the glass tran- sition of E P and PU, there exists another peak at ca. 50°C (see Fig. 2). The peak values differ with

2374 LI AND M A 0

Figure 4. the EP/PU semi-IPN.

SEM micrograph of the fracture surface of

different compositions. When the weight ratio of EP/PU is 70/30, the value reaches the highest. We think that this transition peak corresponds to the interface between EP and PU. The value and the width of this peak respond to the compatibilities of two polymers. The higher the peak value, or the wider the peak, the better the compatibilities. Therefore, we obtain the same conclusion as that of the discussion above.

Within the interface, the self-association of each component causes excess free volume where molecular motion is able to take place relatively easily. Because of the linear order of P U net- work, intermolecular and intramolecular hydro- gen bonds can form in the semi-IPN, as shown by following:

0-0-0

H I

H I

-R-N-c-o-O-c-N-R II 0

II 0

Intermolecular hydrogen bond in PU

-C - C- I I 0

H

I I 0

H

6 ; II II

- R - N - C - O ~ O - C - N - R I H

I H

Intramolecular hydrogen bond between EP and PU

The hydrogen bonds, together with the entangle-

ment and diffusion of polymer chains, serve as physical crosslinking points. The break of these sec- ondary valence bonds accounts for the change in the free volume which is sensitive to the change in tem- perature. Thus, the interface is assumed a glass transition, T,. being dependent on the degree of the compatibility.

Morphology

Figures 3-5 are the SEM photographs of the semi-IPNs free surface morphology and fracture surface morphology. In them, the black areas refer to E P and the white areas refer to PU. I t can be seen that the semi-IPNs embody two-phase con- tinuous structure, as Figure 3 shows. This is also proved by testing their Young’s modulus (see Ta- ble 11).

As we know, the Davies equation suggests a two- phase continuous mechanical model6 as shown by following:

where E, El, and E2 refer to the Young’s modu- lus of the IPN, component 1 and component 2 respectively, and cpl and p2 refer to the volume ratios of two components respectively. Because the relative densities of EP and PU are similar, we use weight ratios instead of volume ratios. Cal- culation shows that the result of EllS is 4.0 and that of (plE:l5 + (p2E:/’ is 4.2. Thus, the data of Young’s modulus meet Davies equation relatively well. This corresponds to the two-phase contin-

Figure 5. the EP/PU semi-IPN.

SEM micrograph of the cellular structure in

SEMI-INTERPENETRATING POLYMER NETWORKS 2375

Table 11. Young’s Modulus of EP/PU Semi-IPNs with Different Weight Compositions

Weight ratio (EP/PU) 100/0 70130 50/50 30/70 0/100 Young’s modulus (MPa) 2374 1015.36 524.67 239.11 70.38

uous structure of EP/PU semi-IPNs we observe in SEM.

Figures 4 and 5 indicate that the system has cellular structure. E P forms the cell wall, while PU forms the cytosome. The cell wall is the main place where E P and PU interpenetrate. More fine cellular structure exists in the PU cy- tosome.

We know that PU is in its rubbery state a t ordinary temperature. Thus, the PU cytosome acts as rubber toughening plastics. They can ini- tiate and stop the developments of crazes and crackles, absorb and diffuse energy of vibration as heat, thus avoiding the fracture of material^.^ Therefore, the formation of a semi-IPN can op- timize the comprehensive properties of polymers because it improves of the compatibility between two polymers.

REFERENCES A N D NOTES

1. M. K. Lindemann, Greenville, et al., (Sun Chemical Co.) U.S. Pat. 4,616,057 (1986).

2. L. H. Sperling, Interpenetrating Polymer Network and Related Materials, National Academy Press, Beijing, 1987.

3. Y. L. Lee, W. H. Ku, et al., J. Polym. Sci. Poly. Chem. Ed., 29(8), 1083 (1991).

4. M. Pate1 and B. Suthar, Angew. Makromol. Chem., 149, 111 (1987).

5. M. Xu and J. Xia, Polym. J., 3(1), 47 (1994). 6. P. Wu, The Mechanism and Technology of Polymer

Blending Modification, Chemical Industry Press, Beijing, 1988.

7. X. Lu, Strength and Collapse of Polymer Materials, Sichuan Education Press, Chengdu, 1988.

Received June 26, 1995 Accepted January 29, 1996