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Structure, mechanism and application of vinyl alcohol oligomers grafted onto poly(3-hydroxybutyrate): a proposal

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e-Polymers
From the journal e-Polymers Volume 14 Issue 6

Abstract

Oligomers of poly(vinyl alcohol) were grafted onto poly(3-hydroxybutyrate) (PHB) by radiation-induced polymerization. The aim of this study was to elucidate the structure of these copolymers using nuclear magnetic resonance (heteronuclear multiple quantum coherence) spectroscopy supported by dynamic light scattering (DLS) and atomic force microscopy (AFM). It was concluded that vinyl alcohol (VA) was grafted onto PHB for the methylenic and methynic sites. A mechanism for the grafting reaction was proposed based on the experimental evidence. AFM and DLS allowed the characterization of the particles obtained from P(HB-g-VA). The prepared materials showed suitable properties for use in drug delivery systems.

Keywords: copolymers; dynamic light scattering (DLS); gamma radiation; poly(3-hydroxybutyrate) (PHB); poly(vinyl alcohol) (PVA)

1 Introduction

The characterization of copolymers, especially graft copolymers, has become one of the most important topics in the advanced materials research (1–3). Graft copolymers obtained from polyhydroxyalkanoates (PHAs) have been extensively studied because of the increasing interest in their properties as biomaterials (4–8). Poly(3-hydroxybutyrate) (PHB) is the first member of the polyhydroxyalkanoate family (9, 10). Much effort has been made to study the graft polymerization of monomers onto PHB using different methods (11–14).

Recently, our research group succeeded in grafting the vinyl acetate monomer onto PHB (15, 16). The polymerization was conducted by γ radiation-induced graft polymerization, using the simultaneous irradiation method. The PHB was used as a fine powder to ensure greater penetration of the monomer and, consequently, to improve the graft degree. Wada et al. (17, 18) also performed a similar work using PHB films; additionally, they reported biodegradability and saponification under different experimental conditions. Some other works have shown a similar trend, but not too many have reported experimental research about the structure of the graft copolymers (19, 20).

Gamma radiation-induced graft polymerization consists in the irradiation of the base polymer in the presence of a monomer. It is a suitable method because of its advantages over the conventional chemistry method, such as the simplicity of the procedure; the reaction can be carried out at room temperature; the polymerization is carried out on the surface and on the internal structures of the base polymer, which permits to study the graft copolymer structure in depth; and the possibility of obtaining high-purity products. It is a simple and fast process, but its drawback, compared to the pre-irradiation method, is the higher levels of homopolymerization that can be achieved during the process, due to the low selectivity of radiation towards the matter. It is also worth mentioning that, compared to the chemical-initiated method, the simultaneous irradiation technique is more complicated because of the complexity of the reactive species formed during the irradiation.

From the characterization tools, nuclear magnetic resonance (NMR) spectroscopy is one of the most important techniques, due to the relevance of the results in the determination of the structure of a polymer and to the support to obtain the mechanism of synthesis (21). For instance, Ye et al. (22) obtained grafted maleic anhydride onto PHB and showed new NMR signals with respect to that of PHB.

The main purpose of the present investigation was to study the structure of the graft copolymer poly(3-hydroxybutyrate)-graft-polyvinyl alcohol [(P(HB-g-VA)] previously obtained at different graft conditions by NMR spectroscopy using heteronuclear multiple bond correlation (HMBC). The current study also investigated the possible application of the material based on the structure. The proposal to use these materials as drug delivery systems was supported by atomic force microscopy AFM and dynamic light scattering (DLS) experiments. Based on experimental evidence, the structure and mechanism of the synthesis were discussed.

2 Experimental

2.1 Materials

Poly(3-hydroxybutyrate) (PHB) (Sigma-Aldrich, Inc. 3050 Spruce Street St. Louis, MO 63103) was purified by precipitation in ethanol from chloroform solutions, prior to use. The monomer, VAc (Merck AG, Germany), was distilled before use. All chemicals were of analytical grade. Because of the instability of vinyl alcohol (VA), poly(vinyl alcohol) (PVA) was made indirectly by polymerization of VAc, followed by hydrolysis of the ester bonds as explained forward.

2.2 Methods

Glass ampules were used for the synthesis reaction. A glass device was specially designed to connect the glass ampules and remove the oxygen. The irradiation experiments were carried out at room temperature. A total of 200 mg of the polymer was added to 0.5 ml of VAc in bulk. The sample VAc (62% w/v) was subjected to 60Co γ rays at a dose and dose rate of 10 kGy and 1.62 kGy/h, respectively. The PHB grafted with VAc was extracted with a Soxhlet using acetone for extensive periods to remove the residues of the VAc monomer and PVAc obtained as collateral products. The graft copolymer P(HB-g-VAc) was dried under vacuum conditions at 40°C to reach a constant weight. The hydrolysis was carried out in methanol solutions of 0.05 mol/L of NaOH for 8–10 h. This product was washed with enough water, filtered and dried under vacuum conditions for further characterization. Each sample was repeated four times to determine the reproducibility under the same graft reaction conditions. Unimportant numerical variation was found when the degree of grafting was measured by thermogravimetric analysis (TGA). In other words, it is indifferent to measure graft degree by TGA or gravimetry.

2.3 Characterization

Both 13C NMR and 1H NMR spectroscopy were carried out using the Mercury 400 BB equipment with a resonance frequency of 400 MHz. It uses the HMBC technique to detect the long-range coupling between proton and carbon samples. The polymeric materials were dissolved in hot CDCl3.

2.4 Topographic images of the particles

Topographic images of the particles were obtained using AFM (Jeol, JSPM 5200 model) in contact and tapping modes separately to obtain desirable images. In all AFM experiments, gold-coated samples were used. An optical microscope was used to help select the desired particles and to direct the position of the AFM tip. Single-particle imaging was performed for four particles from each condition; each group of particles was scanned three times to obtain information on the topography of the particles. All the images were analyzed using the instrument’s software. The 3D image was performed in both phase tapping mode and contact tapping mode for comparison.

The proposal of the reaction mechanism was supported by experimental evidences and by the results of simulation analyses.

2.5 Preparation of P(HB-g-VA) particles

The particles were prepared by the oil/water (o/w) emulsion solvent extraction procedure. One gram of P(HB-g-VA) was dissolved in 50 ml of chloroform at 40°C. The solution was emulsified in an aqueous phase with 4% w/v of PVA as a stabilizer. The mixture was stirred at 1000 rpm overnight to extract chloroform solvent. The particles so obtained were washed with enough water and dried in a vacuum oven for several hours. Finally, the particles were stored under vacuum conditions for further analysis. For the stirring experiments, a special device was designed with a high-shear-rate propeller. The device permits high levels of stirring with a small mixing volume.

2.6 Determination of particle size by DLS

A small amount (about 5 mg) of particles was diluted in 5 ml of solvent (distilled water) to determine the particle size and light scattered intensity Is (which is proportional to the number of concentration of particles) using a DLS apparatus (model BI200SM, Brookhaven Instruments, Brookhaven, NY, USA) equipped with a high-speed digital correlator (PCI-BI9000AT), a solid-state detector and a He-Ne laser of 35 mW (Melles Griot 9167EB-1) as the light source.

3 Results and discussion

3.1 Theoretical calculation of the PVA grafted chain length

Table 1 lists the mass of PHB, P(HB-g-VAc) and PVA (grafted) obtained from the radiation-induced grafting of VAc onto PHB in different solvents. From these results, it can be observed that the mass of PVAc is in the range of 0.013–0.140 g. The PVAc mass was calculated for use in the estimation of the PVA grafted chain length. It is important to note that, although the graft degree [W (%)] of P(HB-g-VA) decreased with respect to P(HB-g-VAc) because of the hydrolysis reaction, the grafting sites that were initially grafted maintained the same quantity. Consequently, the calculation of the chain length over the P(HB-g-VAc) can be directly connected to that of P(HB-g-VA).

Table 1

Calculation of PVAc mass in γ radiation-induced reaction of VAc onto PHB in different solvents.

mPHB (g)mP(HB-g-VAc) (g)mPVAc (g)
P(HB-g-VAc) acetone0.20.21300.013
P(HB-g-VAc) diethyl ether0.20. 21540.0154
P(HB-g-VAc) n-butanol0.20.22620.0262
P(HB-g-VAc) ethyl acetate0.20.22640.0264
P(HB-g-VAc) hexane0.20.20520.0052
P(HB-g-VAc) bulk0.20.33950.1395

The error in the mass determination was ±0.0001 g.

Estimation of the chain length was performed in order to determine the number of PVA chains that were grafted onto the PHB backbone polymer by radiation-induced graft polymerization reaction. The calculation was performed as follows.

First, it is necessary to calculate the chain length of the PHB polymer:

[1]X¯n=M¯nMue=chain length [1]

X¯n=(27,000 g/mol)86 g/mol=314 monomer unitsPHB macromolecule

where X¯n is the degree of polymerization (i.e., the number of monomers in the polymeric chain), which is proportional to the chain length. It is known that each monomeric unit has two carbons that are susceptible to grafting, and it was estimated arbitrarily that about 3% of these were grafted onto the PHB molecules; the estimation was supported by the fact that this is the concentration necessary to generate an NMR signal (15, 23, 24).

[2]ϕ=[γg(%)]∗X¯n [2]

ϕ=0.03×314=9.42 grafted sites per PHB macromolecule.

Table 2 shows the estimation of PHB macromolecules and the monomer molecules grafted, so the total grafted sites are as follows:

Table 2

Estimated calculation of PVA grafted chain length.

M¯n (g/mol)Mass (g)MolesMoleculesaLength
PHB27,0000.26189.6×10-65.84×1018Not applicable
VAc (acetone)860.0131.51×10-490.9×10181.65
VAc (diethyl ether)860.01541.79×10-41.078×10180.02
VAc (n-butanol)860.02623.05×10-42.1×10180.04
VAc (ethyl acetate)860.02643.07×10-41.85×10180.03
VAc (hexane)860.00526.05×10-43.64×10180.06
VAc (bulk)860.13951.6×10-3963×101817.50

Mass.

aMoles×Na (Na=6.02×1023 molecules per mole).

[3]σ=ϕ∗macromolecules of PHB [3]

σ=9.42×5.84×1018=55.01×1018

In the former equation, ϕ refers to the minimum of total grafted sites that can be achieved in a single PHB molecule with a specific average chain length. It also depends on the percentage of grafted sites with respect to the possible sites that can be grafted in the molecule [γg(%)]. In contrast, σ represents the total grafted sites in the graft reaction; this is obtained from the product of the minimum of total grafted sites (ϕ) with the total of PHB macromolecules in the reaction system (no. of moles×NA, where NA is the Avogadro’s number).

From these results, it was deduced that VAc molecules are distributed through the total possible grafted sites per PHB molecule. So, according to this estimation, there exists an average of 3.21 units of VAc per graft. This result is consistent with the existence of a mean of trimers of PVA per probable graft. The length of the graft chain is low because the high viscosity of the media in the graft reaction reduces the possibility that longer oligomer radicals react with PHB radicals and the mass effect prevails. However, it is important to note that longer PVA oligomers can be grafted, although shorter ones are preferred.

3.2 NMR of PHB and grafted copolymers

The base polymer (PHB) and the grafted polymers [P(HB-g-VAc) and P(HB-g-VA)] obtained in bulk were selected for the NMR analyses because they exhibited the greater values of graft degree, so they possess more grafted sites (see Figures 1–3). The NMR studies were extended to heteronuclear multiple quantum coherence (HMQC) in order to detect the long-range coupling between protons and carbons, which permits to verify the graft reaction and elucidate the structure of the graft copolymers as well as the mechanism of grafting.

Figure 1 HMQC coherence NMR spectra of PHB.
Figure 1

HMQC coherence NMR spectra of PHB.

Figure 2 HMQC coherence NMR spectra of P(HB-g-VAc) obtained in bulk.
Figure 2

HMQC coherence NMR spectra of P(HB-g-VAc) obtained in bulk.

Figure 3 HMQC coherence NMR spectra of P(HB-g-VA) obtained in bulk.
Figure 3

HMQC coherence NMR spectra of P(HB-g-VA) obtained in bulk.

In the first attempt, the spectra of the graft copolymers were analyzed. The common signals of 1H NMR at δ=0.5–2 ppm, δ=2.5–3.5 ppm and δ=5–6 ppm were present in the three analyzed samples and they represent the CH3, CH2, and CH signal, respectively. Broad bands were observed in the P(HB-g-VAc) as evidence of polymerization. However, Figure 1 shows that the P(HB-g-VA) 1H NMR seems similar to that of PHB.

In contrast, the 13C NMR spectra of the samples also showed the typical signals of δ=19.23 ppm (CH3), δ=40.27 ppm (CH2), δ=65.97 ppm (CH) and δ=168 ppm attributed to the carbonyl group of PHB. Slight displacements of the original signals were detected for P(HB-g-VAc), which showed δ=20.64 ppm (CH3), δ=38.68 ppm (CH2), δ= (66.91) ppm (CH) and δ=169.85 ppm signals, respectively. This trend was also observed for P(HB-g-VA), but this does not provide a strong evidence to confirm the graft.

Nevertheless, new evidence was found in the 13C NMR spectra of the grafted polymers. A signal at 77 ppm (inside the dichloromethane signal) assigned to the CH carbon probably formed by the introduction of the monomer radical to the methynic or methylenic PHB carbons. This signal has been seen before and strongly supports the graft reaction, while the displacements are in accordance with the new environment of the macromolecule (15).

In addition, the comparison of the HMQC in the three spectra shows that these materials are different. Analysis of the P(HB-g-VAc) shows that the PHB coupling CH*-C* disappeared, which suggests the graft by methynic site ([0-(CH3) Ċ (CH2)]n). Just like that, the original PHB coupling CH2*-C*H3 and CH2*-C*H were not observed in the spectra of the grafted polymer (see Figures 2 and 3). Inspection of the P(HB-g-VAc) spectra also revealed that the coupling CH2*-C*H2 prevailed, while new coupling interactions CH3*-C*H3 came out. Evidence found in the P(HB-g-VAc) spectra suggests the grafting by the methylenic PHB group (ĊH radical obtained from the PHB exposure to γ radiation).

In contrast, the P(HB-g-VA) spectra also showed that the coupling CH3*-C*H and CH2*-C*H2 were intensified with respect to that of P(HB-g-VAc). This shows that, after saponification, the environment of the molecule and, consequently, the C-H interaction around the graft were consolidated (see Figure 3). It is also clear that the copolymer spectrum [P(HB-g-VA] is entirely different to that of PHB. It was concluded that the NMR spectroscopy is in accordance with the grafting of VA onto PHB for both methylenic and methynic sites.

PHB radicals are obtained when PHB is exposed to γ radiation. The exposed molecule contains the unreacted PHB molecules and the PHB radicals that are justified experimentally and theoretically supported by previous investigations (methylenic and methynic radicals) (23, 24) (see the i molecule in Scheme 1). Radical products from the VAc exposure to radiation were also obtained. From the VAc exposition to γ irradiation, the VAc radical showed two conformations, one of which (the CH* radical) is preferred because of its minor activation energy. There is no experimental or theoretical evidence that supports the coupling by CH2* of the VAc radical (see the ii and iii molecules in Scheme 1). The proposal for the initiation reaction includes the VAc first radical (the CH* radical) reaction with the PHB radicals (the CH* or CH2* radical) to form a graft chain radical able to react with any radical in the media (see the iv molecule shown in Scheme 1). In addition, the propagation step proposal is as follows: in this step, the grafted macroradical is formed before reacting with the monomer to form macroradical graft growing chains. In contrast, the homopolymerization is obtained from the reaction of the VAc monomer radical with the monomer itself to bring the macroradical (homopolymer) growing chains (see the vi molecule in Scheme 1). In the last step, the proposal of the termination reaction occurs as it continues: the graft copolymer (shown as the vii molecule in Scheme 1) was probably formed from the coupling of the macroradical graft growing chains and the macroradical homopolymer growing chains (see the v molecule in Scheme 1). Finally, the saponification reaction permits the hydrolysis of the ester groups into the alcohol groups, to produce P(HB-g-VA) molecules (see the viii molecule in Scheme 1). From the proposed mechanism, the branched reaction was suppressed, not because it was not taken into consideration, but because there was no experimental evidence of its formation. It was assumed that other possible reaction products were formed in such small amounts that they could not be observed by the used methods. It was concluded that the mechanism of the graft reaction sustained the obtaining of graft copolymers [P(HB-g-VAc)] accompanied by homopolymerization reactions (PVAc) as well as by the formation of the saponified grafting polymer in basic media [P(HB-g-VA)]. In addition, some important byproducts are not shown because there is not enough experimental evidence to propose their mechanism of synthesis.

Scheme 1 Simplified reaction mechanism of radiation-induced graft of VA oligomers onto PHB.
Scheme 1

Simplified reaction mechanism of radiation-induced graft of VA oligomers onto PHB.

3.3 Surface morphology of the particles obtained by the o/w method

In order to study the surface morphology of the particles obtained by the o/w method [P(HB-g-VA)], the sample surface was imaged in the tapping mode using an atomic force microscope. It was observed that the size of the grafted polymer particles is in the nanometer range (see Figure 4). The force measurements report particles with 30–40 nm in diameter and that are polydisperse. These particles have the appropriate size and shape to be considered as good candidates to encapsulate drugs for an advanced drug delivery system (ADDS).

Figure 4 Three-dimensional AFM of P(HB-g-VA) particles.
Figure 4

Three-dimensional AFM of P(HB-g-VA) particles.

3.4 Analysis of the particle size by DLS

DLS of the P(HB-g-VA) particles was conducted in order to verify the results obtained by AFM. From Figure 5, it can be observed that the average size of the P(HB-g-VA) particles is 19±0.262 nm, but most of the measurements correspond to the range of 19–25 nm. These results are consistent with the AFM images and revealed that the sizes of the particles make them suitable for use in nanotechnology. In addition, very tiny molecules (about 1 nm) were also observed, which were attributed to the small PHB or graft copolymer molecules resulting from the degradation process.

Figure 5 DLS of P(HB-g-VA) particles.
Figure 5

DLS of P(HB-g-VA) particles.

4 Conclusions

According to the NMR results, poly(vinyl alcohol) oligomers can be grafted onto poly(3-hydroxybutyrate) by radiation-induced graft polymerization. The base polymer (PHB) and the grafted polymers [P(HB-g-VAc) and P(HB-g-VA)] obtained in bulk were selected for the HMQC analyses. It was concluded that the VA was grafted onto the PHB for both methylenic and methynic sites. A mechanism for the grafting of VA onto PHB was proposed based on the experimental evidence. AFM allows characterizing the particles obtained from P(HB-g-VA). According to the AFM results, the particles have a diameter of 30–40 nm. In addition, DLS supported the results obtained from the AFM, providing sizes in the range of 19–25 nm. It was concluded that the shape and size of the obtained particles suggest their potential for use in nanotechnology as ADDS of actives.

Corresponding author: Maykel González Torres, PhD, Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México, Campus Juriquilla, Boulevard Juriquilla 3001, Santiago de Querétaro, Querétaro, C.P. 76230, México, Tel.: +52 01-55-5623-4153; Fax: +52 01-55-5623-4165, e-mail:

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Received: 2014-4-25
Accepted: 2014-7-25
Published Online: 2014-9-23
Published in Print: 2014-11-1

©2014 by De Gruyter

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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https://doi.org/10.1515/epoly-2014-0083
Keywords for this article
copolymers; dynamic light scattering (DLS); gamma radiation; poly(3-hydroxybutyrate) (PHB); poly(vinyl alcohol) (PVA)
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Articles in the same Issue

  1. Frontmatter
  2. In this Issue
  3. Editorial
  4. Editorial November 2014
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  6. Controlled ring-opening polymerization of 1,2,6-tricyclic orthoesters of mannose toward size–controlled α-d-mannopyranan
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