Elsevier

Polymer

Volume 41, Issue 26, 15 December 2000, Pages 9265-9272
Polymer

Effect of crosslinking on the mechanical and thermal properties of poly(vinyl alcohol)

https://doi.org/10.1016/S0032-3861(00)00287-1Get rights and content

Abstract

Poly(vinyl alcohol) was crosslinked with hexamethylene diisocyanate in solution. A broad range of degrees of crosslinking, from 1.7 up to 74 mol% of reacted hydroxyl groups, was achieved. The variation of the thermal and mechanical properties of PVA with the crosslinking density show an initial decrease due to the diminution of the crystallinity of the system, caused by the crosslinking. After an abrupt rise at about 20%, the properties tend to level off independently on the increase of the crosslinking. This behaviour is explained as a result of the competitive action of at least three factors during the crosslinking: (i) weakening of the existing physical network due to hydrogen bonding; (ii) formation of a chemical network; and (iii) introduction of flexible moieties. The last factor is closely connected with the specific chemical structure of the crosslinker itself.

Introduction

The chemical resistance and physical properties of poly(vinyl alcohol) (PVA) have led to its broad industrial use. However, the physical properties of PVA depend to a greater extent on the method of preparation than it is the case with other polymers [1]. The final properties are affected by the polymerisation conditions of the parent poly(vinyl acetate) as well as by the hydrolysis conditions, drying and grinding. It is difficult to assign specific physical properties to solid poly(vinyl alcohol) as this designation refers to an array of products, including copolymers of vinyl acetate–vinyl alcohol.

On the other hand, chemical crosslinking of linear polymers may provide feasible routes for the improvement of the mechanical properties and thermal stability [2], [3]. Several crosslinking methods have been published for different uses, since as a rule, all multifunctional compounds capable of reacting with hydroxyl groups can be used to obtain tridimensional networks in PVA [4], [5], [6]. Crosslinked PVA find a very promising application in the preparation of biomedical materials [4] and of magnetic-field-sensitive gels [7].

As far as our knowledge, hexamethylene diisocyanate was scarcely used as a crosslinking agent in PVA. Nevertheless, a comparison of three usual difunctional hardeners showed that only hexamethylene diisocyanate was the most suitable crosslinker [8]. Usually the crosslinking density greatly influences the properties of the final polymer. Therefore the aim of the present work is to offer an extensive study of the evolution of the mechanical and thermal properties of PVA with increasing degree of crosslinking in the broad range from about 2 to 74%, using hexamethylene diisocyanate as a hardener.

Section snippets

Materials

PVA is a commercial product (Gohsenol NL05) with a degree of hydrolysis of 85% and a molecular weight, measured by osmometry, of 22000 g/mol. The tacticity measured by 1H NMR was syndio=21.0%, hetero=53.2% and iso=25.8%. Hexamethylene diisocyanate was purchased from Aldrich and used as received. Dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) (Scharlau) were purified by distillation prior to use.

Crosslinking reactions

A polymer solution was prepared by dissolving 0.81 g of PVA (13.7 mmol

Crosslinking of PVA

The reaction of alcohols with isocyanates to form carbamates is well known and analogous reactions with PVA could be realised. Thus, crosslinking of the PVA may be achieved by reacting hydroxyl groups along the polymer chain with diisocyanates to give urethane crosslinks. The crosslinking reaction of PVA with hexamethylene diisocyanate was performed as indicated in the Section 2. The resulting materials were insoluble in common polar and apolar solvents, indicating their crosslinked nature. The

Conclusions

The detailed study of PVA with different degree of crosslinking has shown that its thermal and mechanical properties suffer an initial decrease, due to the diminution of the crystallinity of the system, caused by the increasing of the crosslinking density. After an abrupt rise at about 20% the properties do not change significantly up to the maximal achieved degree of crosslinking of nearly 74%. This behaviour is explained as a result of the competitive action of at least three factors during

Acknowledgements

The financial support of the CICYT (projects MAT 99-1179 and MAT 98-0961-C02-01) is gratefully acknowledged.

References (21)

  • E Maréchal

    Chemical modification of synthetic polymers

  • R.W Korsmeyer et al.

    J Membr Sci

    (1981)
  • M Zrı́nyi et al.

    Polym Gels Networks

    (1997)
  • K.H Illers

    Eur Polym J (Suppl)

    (1969)
  • M Hidalgo et al.

    Polymer

    (1999)
  • C Fonseca et al.

    Polymer

    (1995)
  • T.P Blomstrom

    Vinyl acetal polymers

  • V Caro et al.

    J Appl Polym Sci

    (1976)
  • V Giménez et al.

    J Appl Polym Sci

    (1997)
There are more references available in the full text version of this article.

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