| Poly(vinyl alcohol)(PVA)has been widely used in medical health cause,cosmetic industry,film packaging industry etc.As a kind of polymer with biocompatibility,biodegradability,safety and environmental protection and high machinability etc.At present,the understanding has just started which about the crystallization process,mechanical and physico-chemical properties of PVA nanofilms.In this paper,we studied the crystallization process of PVA molecules which confined in nanofilms by online IR,then we revealed the mechanism of crystallization,Using IR to calculat the PVA’s crystallinity is accurate,convenient and can reduce the difficulties in the experiment;at the same time we modified PVA films by crystallization,chemical crosslinking,physical crosslinking and a combination of two methods,the modification method of other PVA materials was applied to the PVA nanofilms selectively and we can get PVA nanofilms with high water resistance and good mechanical properties.The main results obtained are as follows:(1)It can improve the crystallinity of PVA nanofilms by heating which is easy to operate.Both water resistance and strength of the films improved greatly.In the process of heating we found that: the crystallization wouldn’t happen until the temperature reached the PVA’s glass transition temperature,the crystallinity of PVA increased during heating and cooling,and the crystallization of cooling made the main contribution to the final crystallinity.Comparing the crystallinity of PVA nanofilms heated under different temperature,we also found that the PVA nanofilms’ crystallinity increased with the rise of temperature within a certain range;to improve the crystallinity the heating temperature should not exceed 190 o C.(2)The water resistance of PVA films after crystallizing was improved from the results of water resistance test,the films wouldn’t dissolve completely while immersed in water for a long time and the water resistance increased with the increase of the crystallinity;and the films’ Young’s modulus increased with the increase of the crystallinity,but the elongation at break was quite the contrary from the nanofilms’ tensile test,the films became hard and brittle.(3)Because heating strengthened the existing network structure,the films’ water resistance was enhanced when heated the films after cross-linking.The PVA nanofilms can continue to becrosslinked after heating,but the degree of crosslinking was low.The films prepared by two different methods can be preserved in the water for more than 30 days,comparing the two kinds of films,we found the water resistance of the nanofilms which heated after cross-linking is better.(4)We can obtain PVA/MA nanofilms that the two components formed physical crosslinking by adding melamine to PVA.It can improve both Young’s modulus and elongation of films even adding a small amount of melamine,from the PVA/MA nanofilms’ tensile test,and the strength and toughness increased with the increase of melamine,we also found the films’ yield distance was long which has important significance in practical application.(5)IR showed that the process of crystallization of PVA/MA nanofilms was consistent with the process of pure PVA’s crystallization when crystallize by heating,the water resistance of PVA/MA nanofilm had also been improved after crystallization;the tensile test’s results of PVA/MA nanofilms with different crystallinity showed that the Young’s modulus increased with the increase of crystallinity and the Young’s modulus can up to 50 times of pure PVA,so we can heat nanofilms appropriately to improve their mechanical properties in practical application. |
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