Thermodynamic Features Of Extended-chain Crystals Of Poly (Butylene Succinate) And Its Copolymers With Butylene Adipate

Thermodynamic parameters are of great importance in the study of polymer crystallization theory.However,a solid and reliable approach to determine the equilibrium melting temperature(T_m~0)and enthalpy of fusion(ΔH_m~0)of polymers is still untouchable.In literatures,T_m~0s of poly(butylene succinate)(PBS)ranging from 127 to151°C were suggested via extrapolation from Gibbs-Thomson(G-T)and/or Hoffman-weeks(H-W)equations by different researchers.In addition,ΔH_m~0 is considered as 110 J/g based on the group contribution method,which is significantly distinct from 210 J/g that was obtained with the dependence of theΔH_m on ralative crystallinity by X-ray.Different considerations among these extrapolation methods result in these conflicting data,which cause the possible error evaluation of undercooling and crystallinity.Consequently,the essential reason for the numerical uncertainty is due to difficulty for preparing crystal samples with enough high melting temperature close to T_m~0.So in tradition,direct measurment of the thermodynamic parameters of PBS is almost impossible.In our previous works,we have successfully developed a simple and effective strategy for obtaining extended-chain crystals(ECC)of PBS with high melting point(～136°C).The continuous thickness along the c-axis was more than 6 times that of folded-chain crystals(FCC)lamellae determined by X-ray detection.Since infinitely large ECC are difficult to obtain in reality,and the extrapolation is still very informative for practical applications.Here,we prepared PBS oligomers ECCs coalescing from inclusion complex(IC),which is made from urea and PBS oligomers of different molecular weights.The crystal thicknesses of oligomer ECCs with constant surface energy are much larger than those of FCCs obtained by conventional procedures such as isothermal crystallization.The ECCs have different melting temperatures and crystal thicknesses(l_c).We found a strong linear relationship between T_m and 1/l_c,which is consistent with G-T equation.By fitting the experimental data of ECCs of PBS oligomers with G-T equation.The as-extrapolated T_m~0 of infinitely large ECC is136.1°C,which is the same as the experimental T_m of ECC of high molecular weight PBS mentioned in our previous work.The slope of G-T diagram is about twice that of G-T fitted with FCCs in previous literatures,which proved that the surface energy of PBS ECC with free-tail structure is higher than that of FCC.On the other hand,the ECC preparation method was also employed to obtain random copolymer ECC of PBSA,whose T_m~0 cannot be determined directly yet.Moreover,since the crystallinity of as-coalesced ECC could not ideally reach 100%,we further used methylamine vapor to etch the remaining amorphous structure on the surface of ECCs.So that melting enthalpy of 100%crystalline sample can be obtained and directly measured,where theΔH_m~0 of PBS has been determined as 133.5±2.8 J/g.This work is the first time realizing physical preparation of copolymer ECCs and thus direct measurement of the thermodynamic parameters.