Project of Master Degree

Investigation of PEG-b-PLLA block copolymer

Contents

• Synthesis
• Depression of the crystallization of the PEG block
• Miscibility of PEG and PLLA blocks
• Block copolymer micelles
• Pseudopolyrotaxanes

Synthesis

Diblock copolymers of polyethylene glycol (PEG) and poly(L-lactic acid) (PLLA) were synthesized by ring-opening polymerization of L-lactide from one end -OH group of PEG, the other one being capped by methoxyl group. The polymerization was catalyzed by stannous octoate (Sn(Oct)₂), which was basically free from racemization so the PLLA blocks in the product are syndiotactic L-lactic acid units. Molecular weight characterization were done by GPC and ¹H NMR. The results from the two methods were close.

Back

Depression of the crystallization of the PEG block

The melt (circle) and crystallization (squre) temperatures of PEG (solid) and PLA (open) blocks in a series of diblock copolymers with varied PLA block length and fixed PEG block length were detected by DSC. The melt and crystallization temperatures of the PLA block increases with increasing block length, while those of PEG block decrease. The two straight lines denote the melt (solid) and crystallization (dash) temperatures of pure PEG. Melt temperature reflects the perfectness of crystallization.

Also the melt enthalpies of PLA block (open squares) increase with decreasing length, while those of PEG block (solid squres) remain uncharged and largely depressed compared with the value of pure PEG (solid line). Melt enthalpy reflects the crystallinity (percent of crystalline phase).

Poly(L-lactic acid) has a higher melt temperature than PEG. So as the copolymer was cooled from melt in DSC, the PLA block crystallized first and freely. As the PEG blocks are all covalently linked to the PLA block, their freedom and space of mobility is confined. So both the crystallinity (ΔH_m) and the perfectness of the crystalline phase (T_m) of the PEG block were largely depressed.

Back

Miscibility of PEG and PLLA blocks

R. Scott (J. Chem. Phys. 1949, 17, 279-284) derived the mixing free energy of two polymers based on P. Flory-Huggins lattice model:

where ϕ_i is the volume fraction of the ith component; m_i is the degree of polymerization of the ith component; χ₁₂ is the interaction parameter.

Considering the equilibrium melt state, T. Nishi et al. (Macromolecules 1975, 8, 909-915) derived the dependent relationship of the melt temperature of a crystalline polymer phase mixed with an amorphous polymer:

where subscript “1″ denotes the amorphous component, while the subscript “2″ denotes the crystalline component. V_iu is the molar volume of the ith component, and ΔH_2u is the molar melt enthalpy of the crystalline component. When m_i are very large (case of polymers), the equation can be reduced to:

In the case of my project, when the PLLA block starts to melt, the PEG block in in melt state (amorphous), so the system is a mixture of crystalline-amorphous polymer mixture. Applying the equation above using such reported values as the densities of amorphous pure PEG and PLLA, T_m⁰ and ΔH_2u of PLLA, the experimental data of 1/T_m of samples with different PLLA block lengths was plotted with the calculated ϕ₁² which, according to the equation, should follow a straight line. Linear fit of the data gave reasonable relativity (R²=0.88). The interaction parameter between PEG melt and PLA melt χ₁₂ was calculated to be -0.29, i.e. they are miscible.

Negative values of interaction parameter χ₁₂ were also reported by various authors for different pairs of miscible polymer components. Amorphous PEG and PLLA were also reported as miscible by other methods. (Citations not shown here. Please ask me for more details.)

Back

Block copolymer micelles

When dissolved in water which is a selectively solvent of the PEG block, the block copolymers form micelles. Under TEM, different topologies were observed for different molecular weight of PLA block in the diblock copolymers.

When the PLLA block is short (W_EG/W_LA=3.89), the copolymer forms spherical micelles (photo above). The largest has a diameter of ca. 120 nm while there are also a large number of tiny micelles (inset) as small as 10 nm.

When the PLLA block is longer (W_EG/W_LA=1.15), rod like micelles with identical thickness of ca. 20nm were formed. The longest rod is ca. 0.5 micron in length, while there are also many tiny rods or dots among the space (arrows).

Theoretically, in crystalline-coil amphiphilic block copolymer micelles (with the crystalline block as the core), it is considered that folding of the crystalline chains consumes energy, which favor larger crystallite volume (smaller number of folds). However, the dissoluble coil segments connect to the crystalline core at the folding points, fewer folds and larger crystalline volume lead to crushing and repulsion of the coil segment which is then unfavorable. The equilibrium micelle morphology is thus the result of the above two competing factors, which are practically controlled, respectively, by temperature and the block lengths. Temperature induce spherical-to-rod transition of PEG-PLLA block copolymer micelles had been reported by T. Fujiwara et al. (Macromolecules 2001, 34, 4043-4050). The results reported here positively confirmed the effect of other factor, block length, on this type of morphology transition.

Back

Pseudopolyrotaxanes

Rotaxane is a type of supramolecules where a ring like molecule is threaded by a rod like molecule through the cavity and locked by end groups at both ends which are larger than the ring cavity. Without the locking end groups, the supramolecule can be called pseudorotaxane. When multiple ring like molecules are threaded on one linear molecules, the prefix “poly” is added to the terms above. See the summary in the scheme.

α-Cyclodextrin, a ring like molecule, easily forms pseudopolyrotaxane with PEG but not PLLA. When the aqueous solution of PEG-b-PLLA block copolymer was mixed with that of α-cyclodextrin, the formation of pseudopolyrotaxane lead to white crystalline precipitation. The structure can be identified as typical channel-type α-cyclodextrin crystalline by XRD spectra, which is independent of the type of polymer included. On the other hand, because of the confinement, no melt or crystallization of PEG can be observed in DSC of the complexed samples, which confirmed the inclusion of PEG chains inside the α-cyclodextrin channels.

Back