Advanced analysis of poly(3-hydroxybutyrate) phases based on vibrational heat capacity
Advanced analysis of poly(3-hydroxybutyrate) phases based on vibrational heat capacity
A. Czerniecka-Kubicka 0 1
I. Zarzyka 0 1
M. Pyda 0 1
0 ATHAS-MP Company , Knoxville, TN 37922 , USA
1 Department of Chemistry, The University of Technology , 35959 Rzeszow , Poland
The new quantitative thermal analysis of semicrystalline poly(3-hydroxybutyrate) (P3HB) phases based on vibrational, solid and liquid heat capacities has been presented. The apparent heat capacity of P3HB was measured using the standard differential scanning calorimetry, and temperature-modulated DSC and quantitative analysis allow for the study of any glass transition, melting/crystallization process and heat capacity of P3HB in the entire range of investigated temperature (183-465 K). The formation and description of phases during thermal processes of P3HB by advanced thermal analysis were examined. The mobile amorphous fraction, degree of crystallinity and rigid amorphous fraction were determined depending on the thermal history of semicrystalline P3HB included after isothermal and non-isothermal crystallization. The experimental, apparent heat capacity of P3HB in the non-equilibrium state was analyzed in reference to the solid and liquid heat capacities and all thermodynamic functions (enthalpy, entropy and Gibbs function) in the equilibrium state.
Poly(3-hydroxybutyrate); Low temperature heat capacity; Vibrational heat capacities; Quantitative thermal analysis
Structure and energy should be examined in order to
describe the physicochemical properties of polymers.
Modern calorimetry and quantitative thermal analysis of
the polymeric materials can provide energetic information
of thermal properties. Measurements of thermal properties
such as the apparent heat capacity and transition
parameters of polymers allow determining the content of phases in
polymeric materials. Polymers can occur in
amorphous (solid or liquid), crystalline or semicrystalline
Most semicrystalline polymers have three phases in the
solid state: one crystalline phase and two amorphous
fractions: the mobile amorphous fraction (MAF) and the
rigid amorphous fraction (RAF). To study polymeric
phases quantitatively, advanced thermal analysis in thermal
characteristics and contents should be applied. Such
analyses are very well described in the literature [1–11].
Typical semicrystalline polymers occur in the
nonequilibrium states, and their thermal properties should be
analyzed to compare to their equilibrium references.
Quantitative thermal analysis allows to establish solid and
liquid baselines of heat capacity, integral thermodynamic
functions and transitions parameters in the equilibrium
state. The first step in this quantitative analysis is to
establish vibrational heat capacity based on
low-temperature experimental data. For poly(3-hydroxybutyrate)
(P3HB), which is considered in this paper, the vibrational,
solid heat capacity was already determined in Ref.  and
it was used for analysis here. In short, the solid, vibrational
baseline was calculated by the assumption that vibrational
motions are only motions contributing to heat capacity at
low temperature, below the glass transition. The vibrational
heat capacity of P3HB was computed based on their group
and skeletal vibrations. The group vibrational heat
capacities were calculated based on the chemical structure and
molecular vibrational motions derived from infrared and
Raman spectroscopy. The skeletal heat capacities were
fitted to a general Tarasov equation. Next, the calculated,
vibrational heat capacity was extended to higher
temperatures, which served as a baseline together with liquid heat
capacity for the new quantitative thermal analysis of the
experimental, apparent heat capacity data of P3HB.
Poly[(R)-3-hydroxybutyrate] (P3HB) is a
polyhydroxyalkanoate (PHA) belonging to the aliphatic polyesters and
is a semicrystalline, isotactic polymer which suffers from
rigidity and brittleness because of their high degree of
crystallinity (50–80 %) , hydrophobicity and lack of
functional groups. Considerable attention has been paid to
the reduction in the degree of crystallinity. Most
commercial plastics are synthetic polymers derived from
petrochemicals. They tend to resist biodegradation.
PHBderived plastics are attractive because they are
compostable and derived from renewables and are
biodegradable. These properties make P3HB a useful thermoplastic
material for utilization in biomedical fields and for low
environmental impact applications . P3HB shows high
potential for medical applications as a degradable implant
material. P3HB has been found to exhibit low toxicity. The
structure of the repeating monomer unit is shown below
(Fig. 1) . The chiral center is defined by the carbon
atom labeled C*.
Previous literature approaches [6, 9, 10, 14–20] to thermal
analyses of P3HB based on the solid heat capacity were
approximated from a linear fit of experimental data below the
glass transition and extended to the higher temperatures also
as the linear function of temperature. In contrast, our new
approach links the low-temperature experimental heat
capacity with vibrational motions of the group and skeleton
of poly(3-hydroxybutyrate) and gives interpretation of Cp on
the microscopic molecular level.
In this paper, the quantitative thermal analysis of
semicrystalline P3HB was performed in order to describe
the thermal properties of P3HB phases based on vibrational
heat capacity. In such a manner, advanced thermal analysis
has been allowed to establish more precisely the contents
of crystalline, mobile and RAFs in the semicrystalline
Poly(3-hydroxybutyrate) is an isotactic, linear and
thermoplastic homopolyester built of 3-hydroxybutyric acid which
was obtained from Biomer (Fig. 1) (Krailling, Germany)
. It is important to note that b-carbon of the monomer in
P3HB is optically active and has R-configuration.
The purity of P3HB was assessed to be around
(96.9 ± 01) % using an elemental analysis by the EA
elemental analyzer 1108 from the Carlo-Erba Company.
The elemental analysis method consists in mineralizing the
sample into inorganic compounds (N2, CO2, H2O) and
determining their composition. The experimental results
show the content of carbon and hydrogen as 54.99 and
7.39 %, respectively, and lack of nitrogen, while the
theoretical contributions of these atoms were calculated as
55.81 and 7.04 %, respectively.
The average molecular weight (Mw) and dispersity (Mw/
Mn) of our P3HB sample were already described in the
literature by Hufenus and co-workers . They had used
gel permeation chromatography (GPC) and chloroform as
the solvent. It was established that value of Mw was around
1.6 9 106 Da and dispersity Mw/Mn around 7.6.
Degradation of P3HB estimated by producer starts above 468 K
(195 C) , and also, this value was confirmed by our
measurement by thermogravimetry (TG).
All experiments were performed using the differential
scanning calorimeters (DSC) a Q1000TM and a TA 2920
from TA Instruments, Inc. (New Castle, DE, USA). These
calorimeters are the heat flux type and are equipped with a
mechanical refrigerator to cool the samples. Measurements
were taken in a nitrogen atmosphere with a constant flow
rate of around 50 mL min-1.
The series of experimental apparent heat capacities were
obtained by standard DSC from the evaluation of measured
heat flow rates at a heating rate of 10 K min-1 after
previous cooling in range of 1–40 K min-1. The temperature
and heat flow rate calibration in the DSC apparatus were
performed using parameters of melting indium
[Tm(onset) = 156.6 C, DHf = 28.45 J g-1 (3.28 kJ mol-1)]
[1, 8, 16]. In order to obtain accurate results, the heat
capacity was calibrated to a sample of sapphire [1, 8]. The
temperature and heat flow were calibrated with indium
metal (Lot J20J32) and heat capacity with sapphire
(970345.901) (disks with mass: 93.449 mg (transparent)
and 95.97 mg (red), obtained from TA Instrument,
970345.901) . Measured accuracy of temperature was
estimated around ±0.1 K.
The mass of the samples used for measurements by DSC
was in the range of 10–30 mg. The heat capacity data were
collected from the second heating run after controlled
cooling or from the first run after isothermal crystallization.
The accuracy of the heat capacity measurements was
estimated to be ±2–3 % or better [1, 2, 14].
Quasi-isothermal temperature-modulated DSC
(qiTMDSC) [1, 3, 9, 24] measurements were taken around the
constant temperature from range of 378–398 K with step
1 K. Isothermal crystallization was studied after cooling
the sample at constant rate of q = 40 K min-1 from the
melt to crystallization temperature (Tc). Kinetics of
crystallization was performed by qi-TMDSC at
Tc = 393 K (120 C) with the modulation amplitude of
0.5 K and period of 60 s. Measured accuracy of heat
capacity by qTDSC was estimated as ±0.5 %.
In order to obtain the semicrystalline samples with
different degrees of crystallinity, the samples were cooled
from the melt to the temperature of 183 K with various
cooling rates (1–40 K min-1) inside of the DSC cell. All
measurements of heat flow and heat capacity were taken on
heating at a rate of 10 K min-1.
Results and discussion
Qualitative thermal analysis of P3HB
Figure 2 shows experimental heat flow rate versus
temperature for received P3HB on heating at rate 10 K min-1 from
183 to 468 K. In Fig. 2, an example of the qualitative thermal
analysis of one from several measurements was presented.
The parameters of glass transition and melting process were
estimated. The glass transition temperature was observed at
283 K (Tg), and the change of heat capacity at Tg was
estimated as DCp = 18.3 J mol-1 K-1 (see Fig. 2, part a). To
integrate the melting peak, the straight baseline was used (see
Fig. 2, part b) and heat of fusion (DHf) was estimated as
9.5 kJ mol-1 which is related to degree of crystallinity,
Wc = 76 % (heat of fusion that equals 12.5 kJ mol-1 of
fully crystalline P3HB was used for calculation). The
melting onset temperature, Tm(onset), at 439.0 K and peak
temperature, [Tm(peak)], at 447.3 K were estimated.
Next, the data presented in Fig. 2 were used for
qualitative and also quantitative thermal analysis to examine
phase content in semicrystalline P3HB, and results are
Quantitative thermal analysis of P3HB
Figure 3 presents the experimental heat capacity, Cp(exp),
versus temperature of semicrystalline P3HB (on the same
data as in Fig. 2) in comparison with the reference baselines
of the solid (vibrational), Cp(solid), and liquid, Cp(liquid),
heat capacities. The solid and liquid baselines of P3HB
were taken from Ref.  where information concerning
these Cp(solid) and Cp(liquid) was described in detail.
Next, the quantitative thermal analysis of presented
P3HB was performed, and results are presented in Figs. 4–
6. Enthalpies, degree of crystallinity, the MAF and the
Fig. 1 Structure of the
repeating unit of P3HB 
RAF as well as quantitative characteristics of glass
transition and melting process of the material were obtained.
Knowing all heat capacities: Cp(exp), Cp(solid) and
Cp(liquid), the thermodynamic function enthalpies
(HðT Þ ¼ R CpdT ) were calculated. Figure 4 illustrates the
experimental enthalpy, Hexp(T), as function of temperature
for semicrystalline P3HB in the frame of the enthalpy of
full amorphous, Ha(T), and crystalline (solid), Hc(T), where
the calculations were based on the data heat capacities
from Fig. 3 as follows: HexpðT Þ ¼ R CpðexpÞdT ; HcðT Þ ¼
R CpðsolidÞdT and HaðT Þ ¼ R CpðliquidÞdT , respectively.
As usual, in Fig. 4, the line of the enthalpy of crystalline
P3HB, Hc(T), occurs much lower than enthalpy of the full
amorphous sample, Ha(T). Enthalpy of solid, Hc(T), jumps
at equilibrium melting temperature, Tm = 470 K, to the
level of the enthalpy of liquid (full amorphous) about heat
of fusion for 100 % crystalline sample DHf
(100 %) = 12.5 kJ mol-1). The enthalpy of full
amorphous, Ha(T), at glass transition temperature Tg = 282.9 K
shows changes of the slope and becomes parallel to the
Hc(T) line below Tg. The experimental enthalpy, H(exp), of
semicrystalline P3HB occurs between both full amorphous,
Ha(T), and full crystal enthalpy, Hc(T).
Next, from these data (Fig. 4), knowing all enthalpies
[Hexp(T), Ha(T) and Hc(T)] as function of temperature, the
degree of crystallinity, Wc(T), was calculated from the
at each different temperature. Result of degree of
crystallinity, Wc(T), as function of temperature for
semicrystalline sample of received P3HB is shown in Fig. 5.
In Fig. 5, degree of crystallinity of P3HB reaches value of
79 % and shows rapid decrease at around 415 K due to
melting process and finally reaches value zero above 450 K.
Also, the change of crystallinity as function of
temperature for semicrystalline polymers can be calculated
directly, using the heat capacities. In this way, Eq. 1 can be
converted into the following form [1, 25, 26]:
where Cp(exp) is the experimental and apparent heat
capacity, that is, the sum of the thermodynamic heat
capacity and the latent heat in the range of melting. The
heat of fusion [DHf(T) = Ha(T) - Hc(T)] is also dependent
on the temperature. It should be noted that the heat capacity
of crystal and glass, below Tg, is usually very similar or
even sometime identical.
Tg = 282.9 K
Tm(onset) = 439.0 K
Tm(peak) = 447.3 K
Tg = 282.9 K
Tm(onset) = 439 K
Fig. 2 Dependence of heat flow rate on
temperature during heating at
10 K min-1 for received sample of
poly(3-hydroxybutyrate) (P3HB). The
inset shows the enlarged region of the
Fig. 3 Dependence of
experimental, apparent heat
capacity, Cp(exp), of P3HB on
heat capacity and Cp(liquid)—
liquid heat capacity
The solution of Eq. 2 gives the degree of crystallinity a
function of temperature Wc(T), and the same result is
presented in Fig. 5.
Depending on thermal history of sample, the glass
transition can be anywhere between (273 and 300) K with
a DCp of about (40–43) J K-1 mol-1 for the
semicrystalline P3HB polymer, respectively. Similarly, the melting
endotherm of semicrystalline P3HB occurs between
434 and 440 K with the fusion heat of about
(7.5–9.6) kJ mol-1, respectively.
Figure 6 shows an example of a quantitative thermal
analysis of an experimental apparent heat capacity for
the semicrystalline P3HB (for the same sample as in
Figs. 2–5). The results show a glass transition, Tg, around
282.9 K and melting process around Tm(onset) = 439 K. As
mentioned earlier, the quantitative thermal analysis of the
experimental data of P3HB was based on the two reference
baselines of the solid and liquid equilibrium heat capacities
[1, 14]. The experimental heat capacity of P3HB below
temperature of 272 K agrees with the solid, calculated
Fig. 4 Enthalpy of
semicrystalline, Hexp(T), full
amorphous, Ha(T), and crystal,
P3HB, versus temperature; Ho is
the integration constant
Fig. 5 Plot of the degree of crystallinity, Wc(T), versus temperature as
calculated for the received sample of P3HB (data from Figs. 3 and 4)
using Eq. (1)
Cp(solid), which means that the low-temperature heat
capacity originates only from vibrational motions. The
deviation starts in the glass transition and above this region
due to contributions of the large-amplitude motions. Next,
at the glass transition temperature Tg = 282.9 K, the
changes in the heat capacity, DCp, for the semicrystalline
sample were estimated to be 8.3 J K-1 mol-1 as the
difference between the solid, vibrational Cp line and a parallel
line obtained by the semicrystalline heat capacity,
Cp(semicrystalline) (see Eq. 3). The change of the heat
capacity for the fully amorphous material at Tg = 282.9 K
was estimated as DCp (100 %) = 39.7 J K-1 mol-1. This
Tg = 282.9 K
Tm° = 470 K
part of the analysis allows determining the MAF, Wa, of the
semicrystalline P3HB from following ratio: Wa = DCp/
DCp (100 %) = 21 %.
The crystallinity of the P3HB sample for the data
presented in Fig. 6 was estimated at 79 %. The crystallinity
function, Wc(T) (see Fig. 5), was used to calculate the
semicrystalline heat capacity line, Cp(semicrystalline),
which separated the true thermodynamic heat capacity
from latent heat in the melting region (see Fig. 6). This
semicrystalline heat capacity, Cp(semicrystalline), was
calculated according to the expression [1, 26–29]:
CpðsemicrystallineÞ ¼ WcðT ÞCpðsolidÞ
Figure 6 shows that the beginning of the melting process
starts at temperature 294.7 K. For semicrystalline P3HB
(Fig. 6), the value of heat of fusion from this advanced
analysis was estimated as 9.5 kJ mol-1. This is the result
of the integration area between experimental, Cp(exp), and
sigmoidal, Cp(semicrystalline), heat capacities from 294.7
to 455 K. The equilibrium heat of fusion for 100 %
crystalline P3HB was taken from the literature  as
DHf = 12.5 kJ mol-1 and was next used for evaluation of
the degree of crystallinity. It should be noted that in the
melting region, two shoulders on the apparent Cp(exp) line
(*400 K and *425 K) occur and probably exist as results
of distribution of different lamella thickness or different
symmetry crystal in the P3HB sample. This subject can be
combined with the study of crystallization/melting related
to molecular weight distribution presented in the literature
[30, 31] and is not considered in this paper.
Tm(onset) = 439 K
Wc = 79 % B
Wa = 21 %
Tg = 282.9 K
Fig. 6 Experimental heat
capacity versus temperature of
poly(3hydroxybutyrate). Data are
shown between the two
baselines of the solid and liquid
heat capacity, Cp(solid) and
Cp(liquid), respectively. In the
temperature 282.9 K, the
change of heat capacity equals
39.7 J K-1 mol-1 for fully
amorphous material (the AC
segment). In the point B, the
degree of amorphous equals
21 %. The temperature of glass
transition was measured at
midlength of the AB distance
Tg = 286 K
Fig. 7 Changes of the experimental apparent heat capacity, Cp(exp),
of P3HB versus temperature, measured on the heating at rate of
10 K min-1 after isothermal crystallization at 393 K for 20 h by
DSC. The results are shown between the two baselines of the solid,
Cp(solid), and liquid, Cp(liquid), heat capacity. At Tg temperature of
286 K, the change of heat capacity equals 41 J K-1 mol-1 for fully
amorphous material (the AD segment). Between the points B and C,
the change of Cp(exp) is associated with the RAF of 7 %, between the
points A and B, with the mobile amorphous phase, MAF, having the
value of 19 %. The temperature of glass transition was measured at
mid-length of the AB distance
Since MAF, Wa, and degree of crystallinity, Wc,
investigated that semicrystalline sample of P3HB presented in
Fig. 6 gave the total value (21 % ? 79 %) of one hundred
percent, any rigid amorphous fraction (RAF) was not
estimated. The analysis presented above was based on the
calculation of crystalline fraction from two-phase model
using Eq. 2, and any possible RAF was included, from
assumption, into solid crystalline phase. Possible rigid
Fig. 8 Time evolution of
experimental heat capacity
(b) of semicrystalline P3HB
during isothermal crystallization
at Tc = 393 K for 700 min.
Curves (a) and (e) indicate the
heat capacity of 100 %
amorphous P3HB and the
vibrational heat capacity of
solid P3HB, respectively. Curve
(d) is the semicrystalline heat
calculated for constant value of
degree of crystallinity 0.74 at
amorphous phase can be relaxed between the beginning of
the deviation of the experimental heat capacity from
Cp(semicrystalline) line (294.7 K) in Fig. 6 until 393 K.
Next, Fig. 7 displays another example of quantitative
thermal analysis of the apparent heat capacity, Cp(exp), of
P3HB versus temperature. Results show the advanced
thermal analysis of the P3HB sample after isothermal
crystallization at 393 K for 20 h from the melt. The Cp(exp) was
obtained on the heating at 10 K min-1 using standard DSC
method. The quantitative thermal analysis of data presented
in Fig. 7 was made similarly to that of the experimental Cp in
Fig. 6. The experimental heat capacity of P3HB was
analyzed in reference to the solid and liquid heat capacities of
P3HB . The phase content was estimated similar as
before for the received P3HB sample. It was ascertained that
amorphous phase content equals 19 % and degree of
crystallinity is 74 %. Results indicate the 7 % presence of the
RAF, WRAF, from the following difference: WRAF =
1 -Wa–Wc. The results above were estimated based on
change of heat capacity at glass transition for mobile
amorphous phase DCpAB ¼ 7:8 J K 1 mol 1, rigid amorphous
phase DCpBC ¼ 2:9 J K 1 mol 1 and for full amorphous
sample DCpAD ¼ 41 J K 1 mol 1. The inset in Fig. 7 presents
the result of calculation of Wc = f(T) according to Eq. 2
where a maximum degree of crystallinity reaches value of
74 %. Next, this crystallinity as the function of temperature,
Wc = f(T), was utilized for the calculation of the expected,
semicrystalline heat capacity, Cp(semicrystalline), baseline
according to Eq. 3. The Cp(semicrystalline) served in the
next step of analysis to calculate the experimental heat of
fusion, DHf(exp) = 9.3 kJ mol-1. Also for the calculation
Cp(cal) = 11.8*(exp)(–t/32.7) + 151.72
Cinf = 151.72 J K–1 mol–1
of degree of crystallinity, in this case, we used the heat of
fusion for full crystal, DHf(100 %), as 12.5 kJ mol-1. In
Fig. 7, the heat capacity marked as the Cp(inf) point was
approximated as reversing component of Cp obtained from
isothermal crystallization at Tc = 393 K for infinity time
using the quasi-isothermal temperature-modulated DSC
The evolution of reversing heat capacity, Cp(reversing)
(b), in time domain during isothermal crystallization of
P3HB at Tc = 393 K (120 C) was performed for an
4000 6000 8000 10,000 12,000 14,000
Heat of fusion/J/mol–1
Fig. 9 Changes of heat capacity (DCp) at glass mobile transition
temperature versus heat of fusion of semicrystalline P3HB with
different thermal history. The (circle points) indicate on two-phase
model of polymer, and the (square points) show three-phase model.
(Color figure online)
Fig. 10 Plot of degree of
amorphous versus degree of
crystallinity of semicrystalline
P3HB with different thermal
history. The (circle points)
indicate that two-phase model
of polymer existed, and the
(square points) show
threephase model, the (square with
cross) and the (circle with
cross) present data for the
received and isothermal
additional study of formation of phases, and result is shown
in Fig. 8.
The reversing heat capacity was measured by
quasiisothermal TMDSC mode with temperature, amplitude
A = 0.5 K and period p = 60 s around 393 K, and the
results were presented in the frame of two baselines of the
vibrational (e) and liquid baselines (a). After approximately
1200 min, the Cp(reversing) of P3HB from the level of
liquid state has researched the level of solid state of
152 J K-1 mol-1. Curve (d) is the semicrystalline heat
capacity, Cp(semicrystalline), calculated for constant value
of degree of crystallinity 0.74 at 393 K. Figure 8 provides
information about formation of the crystalline fraction
(58 %) directly during isothermal crystallization at 393 K.
Final degree of crystallinity was equal to 74 %, meaning
26 % of crystallinity was created between events of
isothermal crystallization and melting on the heating
process. Graphically, this 26 % corresponds to the processes
of crystallization between (c) and (d) lines in Fig. 8.
Additionally, presentations of phase content in the
investigated P3HB (for sample from Fig. 7) are shown in
Figs. 9 and 10.
Figure 9 shows the changes of heat capacity, DCp, at the
glass transition temperature versus the measured heat of
fusion, DHf, for semicrystalline P3HB with different thermal
histories. The experimental data (points) are plotted as
results of qualitative thermal analysis. The circle points
correspond to the existing two phases: Mobile amorphous
phase and crystalline phase give the linear behavior between
DCp and DHf. In turn, the square points show a deviation
from the linearity behavior of the function DCp versus DHf
after advanced analysis
for the received sample
(Wa= 21 %, W = 79 %,WRAF = 0 %)
WRAF = 7 %
Wc = 74 %
after advanced analysis
for the sample after
Wa = 19 %
which indicate a presence of the RAF in the investigated
samples together with the mobile amorphous (RAF) and
crystalline (C) phases. The solid straight line was obtained
from the linear regression of the circle points data and gives
the value of DCp = 41 J K-1 mol-1 at Tg for full
amorphous sample at zero heat of fusion and DHf
(100 %) = 12.5 kJ mol-1 for full crystalline sample at
DCp = 0.
Figure 10 shows the data from Fig. 9 as the degree of
mobile amorphous phase versus degree of crystallinity of
semicrystalline P3HB with different thermal history.
Additionally, data for the received and isothermal crystallized
samples are added to Fig. 9 and are presented in Fig. 10 as a
(square with cross) and a (circle with cross), respectively.
And so the values of the RAF (7 %), MAF (19 %), and
degree of crystallinity (74 %) are shown graphically for the
sample after isothermal crystallization at 393 K analyzed in
Fig. 7 and Wa = 21 %, WRAF = 0 %, and Wc = 79 % for
the received P3HB’s sample from Fig. 6.
Thermal analyses of polymeric phases are often complicated
by irreversible effects, such as broad glass transition,
overlapping enthalpy relaxation with changes of heat capacity at
glass transition, partial crystallinity, reorganization, broad
melting transition and coupling with others phases such as
mobile or rigid amorphous phases and crystalline phase. In
this paper, in order to estimate the content of phases and their
properties more accurately, the advanced thermal analysis of
semicrystalline poly(3-hydroxybutyrate) was performed.
The advanced method requires the use of the equilibrium
thermodynamic references such as in the case of the heat
capacity: the solid and liquid heat capacities that were
applied as baselines to analysis. The solid, vibrational heat
capacity was previously established based on the link of the
low-temperature experimental heat capacity with the
vibrational molecular motion of P3HB. The extended, vibrational
heat capacity to the higher temperature and liquid heat
capacity, baselines, together with the equilibrium
thermodynamic functions and transitions parameters allowed for
analysis of the experimental, apparent heat capacity at a
nonequilibrium state. On this basis, the quantitative thermal
analysis and the phase content of semicrystalline
poly(3hydroxybutyrate) were presented. As a result of this
advanced analysis, the use of the so-called sigmoidal
baseline, Cp(semicrystalline) (see Figs. 6, 7), was demonstrated
for the integration of the melting peak and the separation of
true heat capacity from latent heat in the melting area at
apparent heat capacity of P3HB. A new approach gave 3 %
difference in the estimation of degree of crystallinity
between quantitative (79 %) and qualitative (76 %) analysis
of the same sample of P3HB.
This paper shows that the poly(3-hydroxybutyrate) with
a different thermal history can contain two (crystalline and
mobile amorphous) or three (crystalline, mobile and rigid
amorphous) phases in its structure. Two cases were
presented in detail as examples. In case of the received sample
of P3HB, the content of the crystalline phase was estimated
as Wc = 79 % and mobile amorphous phase as Wa = 21 %
without an occurrence of the rigid amorphous phase. The
second example was illustrated for the
poly(3-hydroxybutyrate) sample after isothermal crystallization that
contained three phases: crystalline phase with Wc = 74 %, the
mobile amorphous phase with Wa = 19 % and the rigid
amorphous phase with WRAF = 7 %. In particular, for the
last case, the advanced analysis was useful for a detection
of a small content of mesophase (RAF), which occurs
between glass transition of MAF and melting point.
The results detailed here for poly(3-hydroxybutyrate)
show that the modern calorimetry and advanced thermal
analysis based on heat capacity and interpretation based on
a molecular (vibrational) motion allow for the study of
thermal properties of phases in metastable polymers and
thus bring the additional approach that improves the
investigations of the well-known polymeric material such
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