| Polyvinyl alcohol(PVA)has been widely used due to its environmentally friendly,cheap,and good water solubility characteristics.The large number of hydrogen bonds in PVA products gives them excellent thermal stability and high mechanical strength,but the presence of hydrogen bonds increases their processing difficulty and limits their service in specific applications.For example,PVA melting point(Tm～220℃)and its thermal decomposition temperature(Td～230℃)are close,making PVA melt processing(dry method)difficult to achieve.PVA has a high glass transition temperature(Tg～70℃),which makes it brittle at room temperature.To address these disadvantages in PVA processing and service,plasticizers are generally introduced to lower their melting points and glass transition temperatures.Glycerol is one of the common plasticizers for PVA,and its addition significantly improves the processability and toughness of PVA products.Therefore,it is a prerequisite to reveal the impact of glycerol on the cohesive structure and chain dynamics of PVA-glycerol system to establish the structure/dynamics-performance coupling relationship of PVA-glycerol system.The amount of plasticizer added is one of the key factors affecting polymer film products.Based on the influence of glycerol with different amounts on the macroscopic service performance(such as melting point and mechanical properties)of PVA film products,the influence of glycerol addition on the cohesive structure and chain dynamics of PVA is revealed through various techniques such as solid-state nuclear magnetic resonance and X-ray scattering.The conclusion is as follows:DSC and mechanical performance tests show that as the glycerol addition amount increases,Tm,crystallization temperature(Tc)and Young’s modulus decrease,while fracture elongation increases.The low-field nuclear magnetic MSE-FID demonstrates the dynamic distribution of glycerol at different temperatures,with a higher flexible phase content of PVA at high temperatures and high glycerol content.Through high-resolution nuclear magnetic resonance,1H NMR,13C CP/MAS,and 13C DP/MAS experiments,the difference in the half-width of the peaks at specific chemical shifts with different glycerol addition amounts is verified,and the half-width of the spectrum decreases with the increase in glycerol addition amount.Through wide-angle X-ray diffraction(WAXS)and small-angle X-ray diffraction(SAXS)combined with low-field solid-state nuclear magnetic spin diffusion experiments,it is shown that glycerol has a significant impact on the amorphous region of PVA,and the plasticizing region of glycerol changes from one-dimensional to two-dimensional morphology with increasing plasticizer content.The selection of plasticizers for specific systems is usually based on the compatibility and persistence of plasticizers in the polymer matrix,the required amount of plasticizer,and the expected physical properties of the film.Bio-polyols have a structure similar to PVA,which can form intermolecular hydrogen bonds with PVA and effectively destroy its structural regularity.Therefore,exploring the effects of different types of bio-polyol plasticizers on PVA not only conforms to the concept of green environmental protection,but also provides a guiding path for the selection criteria of plasticizers.To this end,by selecting six types of bio-alcohol plasticizers,including ethylene glycol,glycerol,erythritol,xylitol,sorbitol,and maltitol,to blend with PVA,the differences in their interactions with PVA were compared to reveal the micromechanism of plasticizer influence on PVA.The size of the interaction parameters between the plasticizer and PVA was compared using various techniques,including DSC(Tm),mechanical testing(breaking elongation and modulus),WAXS(crystallinity),low-field and high-field NMR(chain dynamics distribution),2D NMR(NOE effect),and proton spin diffusion(phase size and morphology).The results show that,except for ethylene glycol,as the molecular volume of the plasticizer increases,the Tm of the PVA-plasticizer system decreases,the breaking elongation decreases,the modulus increases,the crystallinity decreases,the mobile phase content decreases,the NOE effect increases,the phase size first increases and then decreases,and the dynamics dimension decreases.Dimension reflects the uniformity of the plasticizer distribution in PVA.The plasticizing effect is ranked as follows:glycerol>erythritol>xylitol>sorbitol>ethylene glycol>maltitol.In summary,the chain dynamics of the PVAplasticizer system,as well as the relationship between its structure,morphology,and macroscopic properties,were established using the aforementioned methods. |
