Elsevier

Polymer

Volume 42, Issue 13, June 2001, Pages 5755-5761
Polymer

Confined crystallization behavior of PEO in organic networks

https://doi.org/10.1016/S0032-3861(01)00026-XGet rights and content

Abstract

Poly (ethylene oxide) (PEO) and poly (trimethopropane trimethacrylate) (PTMPTMA) interpenetrate networks have been synthesized. The confined crystallization behavior of PEO in the PTMPTMA networks has been investigated by a differential scanning calorimeter and scanning electron microscope. The degree of PEO crystallinity in PEO/PTMPTMA interpenetrate networks reduces with the increase of PTMPTMA. PEO is in an amorphous state when the concentration of PEO is lower than 50% in the interpenetrate networks system. The melting points of crystalline PEO in the networks are lower than that of pure PEO, and the melting point of PEO in the networks is higher and increases with the increase of PEO in the interpenetrate networks. Wide-angle X-ray diffraction results show that the PEO crystallite size perpendicular to the (120) plane is not affected as much as PEO in silica networks.

Introduction

The field of confined liquids, and in particular, that of confined polymers has developed rapidly in the last ten years. Confinement complicates the physics of such films and may alter their properties drastically [1], [2], [3]. Whereas bulk liquids have been investigated rather extensively [4], [5], [6], [7], [8], there are only a handful of quantitative investigations on fluid thin films [9], [10], [11], [12], [13], [14], [15], mainly due to difficulties in preparing samples with well-controlled thickness and interfacial environments. Much attention has been focused on the confined behavior of polymer films, and many interesting results have been reported [16], [17], [18], [19], [20], [21], [22], [23], [24], such as, the viscoelastic dynamics of confined polymer melts, the confinement-induced miscibility in polymer blends, the chain conformation in ultrathin polymer films, and the kinetics of chain organization in ultrathin polymer films, and so on. Computer simulations of polymer melts between impenetrable walls show that chain dimensions parallel to the surfaces are only slightly larger than in the bulk, and that the chain conformation in this direction remains gaussian. In contrast to these ideas, recent self-diffusion measurements for confined polymeric system are consistent with the notion that chain conformation is strongly modified. Similarly, Frank et al. have suggested that chain structure can be significantly affected in ultrathin films (that is thickness D≤100 nm). Preliminary scattering studies by Russell tentatively indicated that chain conformation is modified in the thin film geometry. Similarly, Reich and Cohen found that polymers of high molecular mass exhibited long-range order (up to 10 μm) from the substrate. Based on this, it is clear that even the most fundamental questions regarding the behavior of polymer chains near surfaces and interfaces are poorly understood. In this paper, we present a method to study the behavior of polymers in a confined environment, which differ from the behavior of polymer films less than a micrometer thick.

At present, most of the works on confined polymers are focused on polymer films and ultrathin polymer films. The design of new materials with enhanced properties continues to be a driving force for the investigation of new materials.

Poly (ethylene oxide) (PEO) is a simple and representative linear polymer with interesting behavior both in the blending with other polymers and in the pure state. Because of the fundamental importance and wide applications, many authors have studied the phase structure, morphology and crystallization in the solid state and in solution [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]. These studies have been carried out both in the pure polymer and in its blends, but the behavior of PEO in a confined environment has been less quantitatively understood, when compared with that of PEO in a free environment. The designed networks and the synthesis of the composite samples containing PEO and poly (trimethopropane trimethacrylate) (PTMPTMA) are shown in Scheme 1. During this process PTMPTMA becomes system of networks, and PEO will be embedded in the networks. This approach allows us to study the confined behavior of PEO in a well defined organic network intimately connected to various concentrations of polymer and to investigate the morphology and crystallization of PEO in the confined environment.

Section snippets

Experiment

The PEO here is a commercial product of Polysciences Inc. The Mw and Mn determined by gel permeation chromatography (GPC) are 19 000 and 10 200, respectively, and their ratio is 1.86. The trimethopropane trimethacrylate (TMPTMA) is synthesized in our laboratory. The general procedure for preparing the sample was to dissolve PEO in chloroform at a concentration of about 20 wt%. A measured amount of TMPTMA is mixed with a chloroform solution of PEO and a catalytic amount of benzoperoxide was added

Results and discussion

Fig. 1 shows the DSC curves of PEO/PTMPTMA interpenetrated networks with different ratios on heating. The samples were heated from 0 to 100°C with a heating rate of 10°C/min. In Fig. 1 the samples exhibit no peak when the PEO weight percentage is under 50%. This result indicates that the crystallization of PEO in the networks is strictly confined.

Fig. 2 shows the DSC results of the interpenetrate networks. The weight percentage of crystalline PEO evolved was recorded vs. the concentration of

Conclusions

The present study of PEO/PTMPTMA interpenetrate networks enables us to understand the behavior of PEO in confined environments better. PEO in the networks is not able to crystallize at weight ratio that is lower than 50%. Crystalline PEO in the PTMPTMA networks has a melting temperature different from the PEO in silica networks. The melting temperature of crystalline PEO in the interpenetrate networks system is lower than that of pure PEO. The movement of a PEO molecular chain in the

Acknowledgements

The financial support from the National Natural Science Foundation of China, the National Basic Research Project — Macromolecule Condensed State, and the Fund for Excellent Youth of China are gratefully acknowledged. This work was also subsidized by the Special Funds for Major State Research Projects (No. G 1999064800).

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