Polymorphic packing and dynamics of biodegradable poly(3-hydroxypropionate)

The packing and dynamics of the beta-, gamma-, and delta-forms of poly(3-hydroxypropionate) (P3HP), which represents the basic skeleton of bacterial poly (3-hydroxyalkanoate)s, were investigated by the variable-temperature FTIR and C-13 solid-state NMR measurements (SNMR). Under cooling, most of IR bands in the 1500-750 cm(-1) region were noted to be distinctively blueshifted and enhanced, which should reflect the increased intermolecular interaction and depend on the chain packing of each crystal form. Furthermore, the temperature-dependent splitting of vibration were found to occur at the CH2 bending and rocking for the gamma-form, and at the CH2 bending and C-O-C stretching for the delta-form, while be absent for the beta-form. All the FTIR results indicate that the gamma-form has a stronger intermolecular interaction than the beta-form, although both adopt the all-trans conformation. The IR evidence measured during heating further reveal that the melting of the tightly packed gamma-form would pass through some mesophase, which lacks the regular packing but hold the long-range order along the chains. The delta-form was also found to be tightly packed, and contain at least and most possibly two chains in one unit cell. The CP/MAS C-13 SNMR spectrum of the delta-form was compared with those of the beta- and gamma-forms, and was well explained by combining the gamma-gauche and gamma-eclipsed effects. With considering possible differences in the magnetic dipole-dipole interaction among three crystal forms, the molecular mobilities of crystalline phases were estimated by the values of C-13 spin-lattice relaxation time to rank as delta << gamma << beta. The diversified mobilities of three polymorphic crystalline phases, which is the key to crystalline-structure dependent biodegradability of P3HP, were discussed with considering the packing and conformation.