**2. Experimental**

## **2.1. Materials**

28 Cellulose – Medical, Pharmaceutical and Electronic Applications

**Figure 1.** Scheme for the formation of cellulose allomorphs

**Figure 2.** Conformations of MCCI (A) and MCCII (B)

MCCI (Novacel PH-101, lot 6N608C) was donated by FMC Biopolymers, Philadelphia, PA, USA. Sodium hydroxide (lot 58051305C) and concentrated hydrochloric acid (37%, lot k40039517) were obtained from Carlo Erba, and Merck, respectively. Magnesium stearate (lot 2256KXDS) was purchased from Mallinckrodt Baker and acetaminophen (lot GOH0A01) was obtained from Sigma-Aldrich.

## **2.2. Methods**

## *2.2.1. Preparation of Microcrystalline Cellulose II (MCCII)*

Approximately, 500 g of MCCI was soaked in 3 L of 7.5 N NaOH for 72 h at room temperature. The cellulose II thus obtained was washed with distilled water until it reached neutral pH. The slurry was sequentially passed through 6 (3350 µm), 10 (2000 µm), 24 (711 µm), 40 (425 µm) and 100 (150 µm) mesh screens using an oscillating granulator (Riddhi Pharma Machinery, Gulabnagar, India) when the moisture content was *~*60, 50, 40, 30 and 20%, respectively. The final material was dried in a convection oven at 60C (Model STM 80, Rigger Scientific Inc, Chicago, IL) to a moisture content of less than 5%.

#### *2.2.2. Fourier-Transform Infrared Spectroscopy (FT-IR) Characterization*

Approximately, 1.5 mg of sample was mixed with about 300 mg of dry potassium bromide (previously dried at 110 °C for 4 h) with an agate mortar and pestle. The powdered sample was compressed into a pellet using a 13 mm flat-faced punch a die tooling fitted on a portable press at a dwell time of five minutes. A Perkin Elmer spectrophotometer (Spectrum BX, Perkin Elmer, CA, USA) equipped with the Spectrum software (Perkin Elmer, Inc, CA, USA) was used to obtain the spectrum between 650 to 4000 cm-1. The resolution, interval length and number of scans employed were 16, 2.0 and 16 cm-1, respectively.

#### *2.2.3. Powder X-Rays (P-XRD) Characterization*

Powder X-ray diffraction (P-XRD) measurements were conducted over a 5 to 45º 2θ range using a Rigaku Bench top, diffractometer (Miniflex II, Rigaku Americas, The Woodlands, TX, USA) at 40 kV and 30 mA equipped with monochromatic CuKα (α1=1.5460 Å, α2= 1.54438 Å) X-ray radiation. The sweep speed and step width were 0.5º 2θ/min and 0.008, respectively. The PeakFit software, version 4.12 (SeaSolve Inc, Framingham, MA) was used for the calculation of the areas. The degree of crystallinity was found from the equation [15]:

$$DC = \frac{I\_{\mathbb{C}}}{I\_{r}} \ast 100\% \tag{1}$$

Effect of Polymorphism on the Particle and Compaction Properties of Microcrystalline Cellulose 31

method [] at 25 ±0.5 °C using a Canon-Fenske capillary viscometer (cell size 50) and cupriethylendiamine hydroxide (CUEN) as solvent [16]. The DP was found by the

DP 190 \* [ ]

Where, *N* is the tap number, Vi the initial volume and Vn the volume at the respective tap number. The constant "a" is related to the total volume reduction for the powder bed (compressibility index) and the constant "b" is related to the resistant forces

Samples were fractionated on a RO-TAP sieve shaker (RX29, W.S. Tyler Co., Mentor, OH, USA) using stainless steel 420, 250, 180, 125, 105, 75 µm sieves, stacked together in that order (Fisher Scientific Co., Pittsburgh, PA, USA). Approximately, 50 g of the sample was shaken for 30 min followed by weighing the fractions retained in each sieve. The particle size distributions and the geometric means were found from the log-Normal distributions using

The swelling value is expressed as the ratio of the expanded volume of the powder upon water addition and the initial sample weight. Approximately, 500 mg of the powder was vigorously dispersed in a 10 ml graduate cylinder filled with 10 ml of distilled water at room temperature and the increase in volume of the powder was measured with time [19].

Water sorption isotherms were conducted on a VTI Symmetric Gravimetric Analyzer (Model SGA-100, VTI Corporation, Hialeah, FL), equipped with a chilled mirror dew point analyzer (Model Dewprime IF, Edgetecth ford, MA) at 25 °C. The water activity employed ranged from 0 to 0.9. Water uptake was considered at equilibrium when a sample weight change of no more than 0.01% was reached. Samples were analyzed in triplicate. The nonlinear curve fitting and the resulting parameters were obtained using the Statgraphic software vs. 5 (Warrenton, VA). The Young-Nelson Model (YN) was used for data fitting. This model distinguishes between the tightly bound monolayer, normally condensed externally adsorbed water, and internally absorbed water [20]. In this model, water uptake

> *mA B* ( )

 

The compressibility of the powder was obtained by applying the Kawakita model [17]:

N / V V / V N / a 1 / ab in i (4)

(3)

(5)

relationship:

**2.4. Particle size** 

**2.5. Swelling studies** 

**2.6. Water sorption isotherms** 

is given by equations 5-9:

(friction/cohesion) to compression [18].

the Minitab software (v.16, Minitab®, Inc., State College, PA).

Where IC is the sum of the areas of all crystalline peaks and IT is the area of the amorphous and crystalline regions.

#### **2.3. Powder properties**

The microphotographs were taken on an optical microscope (BM-180, Boeco, Germany) coupled with a digital camera (S8000fd, Fujifilm Corp., Japan) at 700X magnification. The true density was determined on a helium picnometer (AccuPyc II 1340, Micromeritics, USA) with *~*2 g of sample. Bulk density was determined by the ratio of 20 g of sample divided the measured volume. Tap density was measured directly from the final volume of the tapped sample obtained from the AUTO-TAP analyzer (AT-2, Quantachrome instruments, USA). Flow rate was obtained by measuring the time for ~20 g of sample to pass through a glass funnel (13 mm diam). Porosity () of the powder was determined from the equation:

$$\varepsilon = 1 - \left(\frac{\rho\_{bulk}}{\rho\_{true}}\right) \tag{2}$$

Where, , bulk, and true are the porosity, bulk density and true density of the powder, respectively. The degree of polymerization (DP) was obtained by the intrinsic viscosity method [] at 25 ±0.5 °C using a Canon-Fenske capillary viscometer (cell size 50) and cupriethylendiamine hydroxide (CUEN) as solvent [16]. The DP was found by the relationship:

$$\text{DP} = 190^\* \left[ \eta \right] \tag{3}$$

The compressibility of the powder was obtained by applying the Kawakita model [17]:

$$\mathbf{N} / \left[ \left( \mathbf{V\_i} - \mathbf{V\_n} \right) / \mathbf{V\_l} \right] = \mathbf{N} / \mathbf{a} + \mathbf{1} / \mathbf{ab} \tag{4}$$

Where, *N* is the tap number, Vi the initial volume and Vn the volume at the respective tap number. The constant "a" is related to the total volume reduction for the powder bed (compressibility index) and the constant "b" is related to the resistant forces (friction/cohesion) to compression [18].

#### **2.4. Particle size**

30 Cellulose – Medical, Pharmaceutical and Electronic Applications

*2.2.3. Powder X-Rays (P-XRD) Characterization* 

and crystalline regions.

**2.3. Powder properties** 

Pharma Machinery, Gulabnagar, India) when the moisture content was *~*60, 50, 40, 30 and 20%, respectively. The final material was dried in a convection oven at 60C (Model STM 80,

Approximately, 1.5 mg of sample was mixed with about 300 mg of dry potassium bromide (previously dried at 110 °C for 4 h) with an agate mortar and pestle. The powdered sample was compressed into a pellet using a 13 mm flat-faced punch a die tooling fitted on a portable press at a dwell time of five minutes. A Perkin Elmer spectrophotometer (Spectrum BX, Perkin Elmer, CA, USA) equipped with the Spectrum software (Perkin Elmer, Inc, CA, USA) was used to obtain the spectrum between 650 to 4000 cm-1. The resolution, interval

Powder X-ray diffraction (P-XRD) measurements were conducted over a 5 to 45º 2θ range using a Rigaku Bench top, diffractometer (Miniflex II, Rigaku Americas, The Woodlands, TX, USA) at 40 kV and 30 mA equipped with monochromatic CuKα (α1=1.5460 Å, α2= 1.54438 Å) X-ray radiation. The sweep speed and step width were 0.5º 2θ/min and 0.008, respectively. The PeakFit software, version 4.12 (SeaSolve Inc, Framingham, MA) was used for the calculation of the areas. The degree of crystallinity was found from the equation [15]:

> *<sup>C</sup>* \* 100% *T*

Where IC is the sum of the areas of all crystalline peaks and IT is the area of the amorphous

The microphotographs were taken on an optical microscope (BM-180, Boeco, Germany) coupled with a digital camera (S8000fd, Fujifilm Corp., Japan) at 700X magnification. The true density was determined on a helium picnometer (AccuPyc II 1340, Micromeritics, USA) with *~*2 g of sample. Bulk density was determined by the ratio of 20 g of sample divided the measured volume. Tap density was measured directly from the final volume of the tapped sample obtained from the AUTO-TAP analyzer (AT-2, Quantachrome instruments, USA). Flow rate was obtained by measuring the time for ~20 g of sample to pass through a glass

funnel (13 mm diam). Porosity () of the powder was determined from the equation:

1 *bulk true* 

Where, , bulk, and true are the porosity, bulk density and true density of the powder, respectively. The degree of polymerization (DP) was obtained by the intrinsic viscosity

 

*<sup>I</sup>* (1)

(2)

*<sup>I</sup> DC*

Rigger Scientific Inc, Chicago, IL) to a moisture content of less than 5%.

*2.2.2. Fourier-Transform Infrared Spectroscopy (FT-IR) Characterization* 

length and number of scans employed were 16, 2.0 and 16 cm-1, respectively.

Samples were fractionated on a RO-TAP sieve shaker (RX29, W.S. Tyler Co., Mentor, OH, USA) using stainless steel 420, 250, 180, 125, 105, 75 µm sieves, stacked together in that order (Fisher Scientific Co., Pittsburgh, PA, USA). Approximately, 50 g of the sample was shaken for 30 min followed by weighing the fractions retained in each sieve. The particle size distributions and the geometric means were found from the log-Normal distributions using the Minitab software (v.16, Minitab®, Inc., State College, PA).

#### **2.5. Swelling studies**

The swelling value is expressed as the ratio of the expanded volume of the powder upon water addition and the initial sample weight. Approximately, 500 mg of the powder was vigorously dispersed in a 10 ml graduate cylinder filled with 10 ml of distilled water at room temperature and the increase in volume of the powder was measured with time [19].

#### **2.6. Water sorption isotherms**

Water sorption isotherms were conducted on a VTI Symmetric Gravimetric Analyzer (Model SGA-100, VTI Corporation, Hialeah, FL), equipped with a chilled mirror dew point analyzer (Model Dewprime IF, Edgetecth ford, MA) at 25 °C. The water activity employed ranged from 0 to 0.9. Water uptake was considered at equilibrium when a sample weight change of no more than 0.01% was reached. Samples were analyzed in triplicate. The nonlinear curve fitting and the resulting parameters were obtained using the Statgraphic software vs. 5 (Warrenton, VA). The Young-Nelson Model (YN) was used for data fitting. This model distinguishes between the tightly bound monolayer, normally condensed externally adsorbed water, and internally absorbed water [20]. In this model, water uptake is given by equations 5-9:

$$m = A(\theta + \beta) + B\Psi \tag{5}$$

$$\theta = \frac{a\_w}{a\_w + (1 - a\_w)E} \tag{6}$$

$$
\Psi = a\_{\underline{\boldsymbol{a}}} \theta \tag{7}
$$

Effect of Polymorphism on the Particle and Compaction Properties of Microcrystalline Cellulose 33

calculated by dividing the bulk density with the true density [26]. The strain rate sensitivity (SRS) was found by the percentage change of the Py resulted from 1 and 0.03 compact/s

It was determined on a VanKel hardness tester (UK 200, VanKel, Manasquan, NJ, USA). Each compact was placed between the platens and the crushing force was then measured. The radial tensile strength (TS) values were obtained according to the Fell and Newton

<sup>2</sup> *TS*

Where, F is the crushing force (N) needed to break the compact into two halves, D is the diameter of the compact (mm), and *t* is the compact thickness (mm). The crosshead speed of

Tablets containing different levels of acetaminophen (25, 50, 75, 85 or 95%) and a poorly compressible drug, were prepared and their crushing strength was determined. Acetaminophen and the test excipient were mixed in a V-Blender for 30 min and then compressed on a single punch tablet press at 120 MPa and a dwell time of 30 s. Samples

Lubricant sensitivity was assessed by mixing powders with magnesium stearate at the 99:1 weight ratio in a V-blender (Riddhi Pharma Machinery, Gulabnagar, India) for 30 min. Tablets were prepared using a single punch tablet press at 120 MPa and a dwell time of 30 s. The lubricant sensitivity was expressed as the lubricant sensitivity ratio (LSR) according to

> 0 0

*H Hlub LSR H*

Where, H0 and Hlub are the crushing strengths of tablets prepared without and with

The friability test was performed on a friabilator (FAB-25, Logan Instruments Corp., NJ, USA) at 25 rpm for 4 min. An amount of ~6.5 g of compacts made at 150 MPa, each weighing ∼500 mg, was tested in a friabilator. Compacts were then dusted and reweighed.

lubricant, respectively. Samples were analyzed in triplicate.

The percentage weight loss was taken as friability.

*DH* 

(12)

(13)

speeds, respectively.

equation [27]:

**2.9. Compact tensile strength** 

the left moving platen was 3.5 mm/s.

**2.11. Lubricant Sensitivity (LSR)** 

**2.10. Dilution potential** 

were analyzed in triplicate.

**2.12. Compact friability** 

the equation:

$$\beta = -\frac{E a\_w}{E - (E - 1) a\_w} + \left(\frac{E^2}{E - 1}\right) \ln \frac{E - (E - 1) a\_w}{E} - (E + 1) \ln(1 - a\_w) \tag{8}$$

$$E = e^{-(H1-Hl)/RT} \tag{9}$$

Where, m, , , and B correspond to the total fractional moisture content, the fraction of molecules cover by monolayer, the fraction covered by a layer 2 or more molecules thick, and the amount of absorbed water in the multilayer. H1 is the heat of adsorption of water bound to the surface, HL the heat of condensation, R is the gas constant (8.31 J/Kmol), and T the temperature. A and B are dimensionless constants related to the fraction of adsorbed and absorbed water on the polymer, respectively. E is the equilibrium constant between the monolayer and liquid water. The product A is related to the amount of water in the monolayer and A(+B) is the externally adsorbed moisture during the sorption phase. B is the amount of moisture absorbed during the sorption phase [21].

#### **2.7. Tableting properties**

Compacts of ∼500 mg each were made on a single punch tablet press (Compac 060804, Indemec Ltd, Itagui, Colombia) coupled with a load cell (Model LCGD-10K, Omega Engineering, Inc., Stamford, CT) using flat-faced 13 mm punches and die tooling for 1 and 30 s. Pressures ranged from ∼35 to ∼190 MPa. Forces were measured on a strain gauge meter (Model DPiS8-EI, Omega Engineering, Inc., Stamford, CT). Compact heights were measured immediately after production and after 5 days of storage to measure the elastic recovery of the material.

#### **2.8. Compressibility analysis**

The natural logarithm of the inverse of compact porosity, [-ln(ε)], was plotted against compression pressure (P) to construct the Heckel plot [22, 23]. The slope (m) of the linear region of this curve is inversely related to the material yield pressure (Py), which is a measurement of its plasticity [24]. Thus, a low Py (usually values <100 MPa) indicates a high ductile deformation mechanism upon compression. The Heckel model is given by:

$$-\ln \mathcal{z} = mP + A \tag{11}$$

Where, A is the intercept obtained by extrapolating the linear region to zero pressure. Other parameters useful in assessing compressibility are D0, Da, and Db, which are related to initial powder packing/densification, total compact densification, and particle rearrangement/fragmentation at the initial compaction stage, respectively [25]. D0 was calculated by dividing the bulk density with the true density [26]. The strain rate sensitivity (SRS) was found by the percentage change of the Py resulted from 1 and 0.03 compact/s speeds, respectively.

#### **2.9. Compact tensile strength**

32 Cellulose – Medical, Pharmaceutical and Electronic Applications

**2.7. Tableting properties** 

recovery of the material.

**2.8. Compressibility analysis** 

(1 ) *w w w a a aE*

<sup>2</sup> ( 1) ln ( 1)ln(1 ) ( 1) 1

*Ea <sup>E</sup> EE a E a*

(6)

*w*

(8)

(7)

( 1 )/ *H Hl RT E e* (9)

*mP A* (11)

*EE a E E*

*w*

the amount of moisture absorbed during the sorption phase [21].

*w w*

Where, m, , , and B correspond to the total fractional moisture content, the fraction of molecules cover by monolayer, the fraction covered by a layer 2 or more molecules thick, and the amount of absorbed water in the multilayer. H1 is the heat of adsorption of water bound to the surface, HL the heat of condensation, R is the gas constant (8.31 J/Kmol), and T the temperature. A and B are dimensionless constants related to the fraction of adsorbed and absorbed water on the polymer, respectively. E is the equilibrium constant between the monolayer and liquid water. The product A is related to the amount of water in the monolayer and A(+B) is the externally adsorbed moisture during the sorption phase. B is

Compacts of ∼500 mg each were made on a single punch tablet press (Compac 060804, Indemec Ltd, Itagui, Colombia) coupled with a load cell (Model LCGD-10K, Omega Engineering, Inc., Stamford, CT) using flat-faced 13 mm punches and die tooling for 1 and 30 s. Pressures ranged from ∼35 to ∼190 MPa. Forces were measured on a strain gauge meter (Model DPiS8-EI, Omega Engineering, Inc., Stamford, CT). Compact heights were measured immediately after production and after 5 days of storage to measure the elastic

The natural logarithm of the inverse of compact porosity, [-ln(ε)], was plotted against compression pressure (P) to construct the Heckel plot [22, 23]. The slope (m) of the linear region of this curve is inversely related to the material yield pressure (Py), which is a measurement of its plasticity [24]. Thus, a low Py (usually values <100 MPa) indicates a high

Where, A is the intercept obtained by extrapolating the linear region to zero pressure. Other parameters useful in assessing compressibility are D0, Da, and Db, which are related to initial powder packing/densification, total compact densification, and particle rearrangement/fragmentation at the initial compaction stage, respectively [25]. D0 was

ductile deformation mechanism upon compression. The Heckel model is given by:

ln

*<sup>w</sup> a* 

> It was determined on a VanKel hardness tester (UK 200, VanKel, Manasquan, NJ, USA). Each compact was placed between the platens and the crushing force was then measured. The radial tensile strength (TS) values were obtained according to the Fell and Newton equation [27]:

$$\text{TS} = \frac{2\sigma}{\pi DH} \tag{12}$$

Where, F is the crushing force (N) needed to break the compact into two halves, D is the diameter of the compact (mm), and *t* is the compact thickness (mm). The crosshead speed of the left moving platen was 3.5 mm/s.

#### **2.10. Dilution potential**

Tablets containing different levels of acetaminophen (25, 50, 75, 85 or 95%) and a poorly compressible drug, were prepared and their crushing strength was determined. Acetaminophen and the test excipient were mixed in a V-Blender for 30 min and then compressed on a single punch tablet press at 120 MPa and a dwell time of 30 s. Samples were analyzed in triplicate.

#### **2.11. Lubricant Sensitivity (LSR)**

Lubricant sensitivity was assessed by mixing powders with magnesium stearate at the 99:1 weight ratio in a V-blender (Riddhi Pharma Machinery, Gulabnagar, India) for 30 min. Tablets were prepared using a single punch tablet press at 120 MPa and a dwell time of 30 s. The lubricant sensitivity was expressed as the lubricant sensitivity ratio (LSR) according to the equation:

$$LSR = \frac{H\_0 - H\_{\text{lab}}}{H\_0} \tag{13}$$

Where, H0 and Hlub are the crushing strengths of tablets prepared without and with lubricant, respectively. Samples were analyzed in triplicate.

#### **2.12. Compact friability**

The friability test was performed on a friabilator (FAB-25, Logan Instruments Corp., NJ, USA) at 25 rpm for 4 min. An amount of ~6.5 g of compacts made at 150 MPa, each weighing ∼500 mg, was tested in a friabilator. Compacts were then dusted and reweighed. The percentage weight loss was taken as friability.

## **2.13. Compact disintegration**

Tablets, each weighing ~500 mg, were made on a single punch tablet press (060804, Indumec, Itagui, Columbia) using a 13 mm round, flat-faced punches and die set. Five replicates were tested in distilled water at 37 C employing a Hanson disintegrator (39-133-115, Hanson Research Corporation, Northridge, CA, USA) operating at 30 strokes/min.

Effect of Polymorphism on the Particle and Compaction Properties of Microcrystalline Cellulose 35

1653 1377 2922

1652 2899

 3000 2000 1000 Wavenumbers (cm-1)

605

607

1066

1375

1063

MCCI

MCCII

**Figure 3.** FT-IR of microcrystalline cellulose allomorphs

3423

3404

3757

**Figure 4.** Powder XRD of microcrystalline cellulose allomorphs

larger tendency to have high frequencies in the low particle size region.

Figure 5 shows micrographs depicting particle morphologies. It seems to be that the polymorphic transformation had little effect on the morphology of these particles. Both, MCCI and MCCII consisted of aggregated and irregularly-shaped particles with rough surfaces and sharp edges. However, elongated particles were more predominant for MCCI. Table 1 lists the powder properties of these materials. Since the polymorphic transformation did not cause major morphological changes in the particles, the mean particle size of the two polymorphs was comparable. The particle size distribution is depicted in Figure 6. In this case, both materials showed a positively skewed distribution, but MCCII had a slightly

2-Theta 5 10 20 30 40

**3.3. Powder properties** 

0

20

22

24

26

28

30

%Transmittance

32

34

36

38
